test: Remove `system_tests/run_command` runtime dependencies

diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index d5f2776a..0bfcc244 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -61,6 +61,7 @@ add_executable(test_bitcoin
   miniminer_tests.cpp
   miniscript_tests.cpp
   minisketch_tests.cpp
+  mock_process.cpp
   multisig_tests.cpp
   bip328_tests.cpp
   net_peer_connection_tests.cpp
@@ -188,6 +189,8 @@ function(add_boost_test source_file)
   list(TRANSFORM test_suite_macro
     REPLACE "(BOOST_FIXTURE_TEST_SUITE|BOOST_AUTO_TEST_SUITE)\\(" ""
   )
+  # The mock_process test suite does not contain unit tests.
+  list(REMOVE_ITEM test_suite_macro "mock_process")
   foreach(test_suite_name IN LISTS test_suite_macro)
     add_test(NAME ${test_suite_name}
       COMMAND test_bitcoin --run_test=${test_suite_name} --catch_system_error=no --log_level=test_suite -- DEBUG_LOG_OUT
diff --git a/src/test/mock_process.cpp b/src/test/mock_process.cpp
new file mode 100644
index 00000000..c9100c9f
--- /dev/null
+++ b/src/test/mock_process.cpp
@@ -0,0 +1,40 @@
+// Copyright (c) 2025-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#include <boost/test/unit_test.hpp>
+
+#include <iostream>
+#include <string>
+
+BOOST_AUTO_TEST_SUITE(mock_process, *boost::unit_test::disabled())
+
+BOOST_AUTO_TEST_CASE(valid_json, *boost::unit_test::disabled())
+{
+    std::cout << R"({"success": true})" << std::endl;
+}
+
+BOOST_AUTO_TEST_CASE(nonzeroexit_nooutput, *boost::unit_test::disabled())
+{
+    BOOST_FAIL("Test unconditionally fails.");
+}
+
+BOOST_AUTO_TEST_CASE(nonzeroexit_stderroutput, *boost::unit_test::disabled())
+{
+    std::cerr << "err\n";
+    BOOST_FAIL("Test unconditionally fails.");
+}
+
+BOOST_AUTO_TEST_CASE(invalid_json, *boost::unit_test::disabled())
+{
+    std::cout << "{\n";
+}
+
+BOOST_AUTO_TEST_CASE(pass_stdin_to_stdout, *boost::unit_test::disabled())
+{
+    std::string s;
+    std::getline(std::cin, s);
+    std::cout << s << std::endl;
+}
+
+BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/system_tests.cpp b/src/test/system_tests.cpp
index f4490dec..c5eac1cb 100644
--- a/src/test/system_tests.cpp
+++ b/src/test/system_tests.cpp
@@ -14,12 +14,20 @@
 #include <util/subprocess.h>
 #endif // ENABLE_EXTERNAL_SIGNER
 
+#include <boost/cstdlib.hpp>
 #include <boost/test/unit_test.hpp>
 
+#include <string>
+
 BOOST_FIXTURE_TEST_SUITE(system_tests, BasicTestingSetup)
 
 #ifdef ENABLE_EXTERNAL_SIGNER
 
+static std::vector<std::string> mock_executable(std::string name)
+{
+    return {boost::unit_test::framework::master_test_suite().argv[0], "--log_level=nothing", "--report_level=no", "--run_test=mock_process/" + name};
+}
+
 BOOST_AUTO_TEST_CASE(run_command)
 {
     {
@@ -27,11 +35,7 @@ BOOST_AUTO_TEST_CASE(run_command)
         BOOST_CHECK(result.isNull());
     }
     {
-#ifdef WIN32
-        const UniValue result = RunCommandParseJSON({"cmd.exe", "/c", "echo", "{\"success\":", "true}"}); // The command is intentionally split "incorrectly", to exactly preserve previous behavior. This is due to the cmd.exe internal echo quoting strings with spaces in it, unlike the normal 'echo' below.
-#else
-        const UniValue result = RunCommandParseJSON({"echo", "{\"success\": true}"});
-#endif
+        const UniValue result = RunCommandParseJSON(mock_executable("valid_json"));
         BOOST_CHECK(result.isObject());
         const UniValue& success = result.find_value("success");
         BOOST_CHECK(!success.isNull());
@@ -48,45 +52,32 @@ BOOST_AUTO_TEST_CASE(run_command)
     }
     {
         // Return non-zero exit code, no output to stderr
-#ifdef WIN32
-        const std::vector<std::string> command = {"cmd.exe", "/c", "exit 1"};
-#else
-        const std::vector<std::string> command = {"false"};
-#endif
+        const std::vector<std::string> command = mock_executable("nonzeroexit_nooutput");
         BOOST_CHECK_EXCEPTION(RunCommandParseJSON(command), std::runtime_error, [&](const std::runtime_error& e) {
             const std::string what{e.what()};
-            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned 1: \n", util::Join(command, " "))) != std::string::npos);
+            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned %d: \n", util::Join(command, " "), boost::exit_test_failure)) != std::string::npos);
             return true;
         });
     }
     {
         // Return non-zero exit code, with error message for stderr
-#ifdef WIN32
-        const std::vector<std::string> command = {"cmd.exe", "/c", "echo err 1>&2 && exit 1"};
-#else
-        const std::vector<std::string> command = {"sh", "-c", "echo err 1>&2 && false"};
-#endif
+        const std::vector<std::string> command = mock_executable("nonzeroexit_stderroutput");
         const std::string expected{"err"};
         BOOST_CHECK_EXCEPTION(RunCommandParseJSON(command), std::runtime_error, [&](const std::runtime_error& e) {
             const std::string what(e.what());
-            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned %s: %s", util::Join(command, " "), 1, "err")) != std::string::npos);
+            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned %s: %s", util::Join(command, " "), boost::exit_test_failure, "err")) != std::string::npos);
             BOOST_CHECK(what.find(expected) != std::string::npos);
             return true;
         });
     }
     {
         // Unable to parse JSON
-#ifdef WIN32
-        const std::vector<std::string> command = {"cmd.exe", "/c", "echo {"};
-#else
-        const std::vector<std::string> command = {"echo", "{"};
-#endif
-        BOOST_CHECK_EXCEPTION(RunCommandParseJSON(command), std::runtime_error, HasReason("Unable to parse JSON: {"));
+        BOOST_CHECK_EXCEPTION(RunCommandParseJSON(mock_executable("invalid_json")), std::runtime_error, HasReason("Unable to parse JSON: {"));
     }
 #ifndef WIN32
     {
         // Test stdin
-        const UniValue result = RunCommandParseJSON({"cat"}, "{\"success\": true}");
+        const UniValue result = RunCommandParseJSON(mock_executable("pass_stdin_to_stdout"), "{\"success\": true}");
         BOOST_CHECK(result.isObject());
         const UniValue& success = result.find_value("success");
         BOOST_CHECK(!success.isNull());
diff --git a/test/lint/lint-includes.py b/test/lint/lint-includes.py
index cccaec73..70c4e0e2 100755
--- a/test/lint/lint-includes.py
+++ b/test/lint/lint-includes.py
@@ -21,6 +21,7 @@ EXCLUDED_DIRS = ["contrib/devtools/bitcoin-tidy/",
                 ] + SHARED_EXCLUDED_SUBTREES
 
 EXPECTED_BOOST_INCLUDES = [
+                           "boost/cstdlib.hpp",
                            "boost/multi_index/detail/hash_index_iterator.hpp",
                            "boost/multi_index/hashed_index.hpp",
                            "boost/multi_index/identity.hpp",