test: Remove `mock_process.cpp`

diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index 97a034de..b5a12f72 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -64,7 +64,6 @@ 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
@@ -192,8 +191,6 @@ 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 -- -printtoconsole=1
diff --git a/src/test/mock_process.cpp b/src/test/mock_process.cpp
deleted file mode 100644
index c9100c9f..00000000
--- a/src/test/mock_process.cpp
+++ /dev/null
@@ -1,40 +0,0 @@
-// 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 408666d6..1b1d6bc7 100644
--- a/src/test/system_tests.cpp
+++ b/src/test/system_tests.cpp
@@ -11,33 +11,65 @@
 #include <univalue.h>
 #include <util/string.h>
 
+#include <cstdlib>
+#include <iostream>
+#include <string_view>
+
 #ifdef ENABLE_EXTERNAL_SIGNER
 #include <util/subprocess.h>
 #endif // ENABLE_EXTERNAL_SIGNER
 
-#include <boost/cstdlib.hpp>
 #include <boost/test/unit_test.hpp>
 
 #include <string>
 
+namespace {
+// When set in the environment, test_bitcoin acts as a mock subprocess for the
+// run_command test below instead of running unit tests.
+constexpr const char* MOCK_PROCESS_ENV = "BITCOIN_TEST_MOCK_PROCESS";
+
+const bool g_maybe_run_mock_dispatcher_before_main{[]() {
+    const char* name = std::getenv(MOCK_PROCESS_ENV);
+    if (!name) return false;
+    const std::string_view n{name};
+    if (n == "valid_json") {
+        std::cout << R"({"success": true})" << std::endl;
+        std::_Exit(EXIT_SUCCESS);
+    }
+    if (n == "nonzeroexit_nooutput") {
+        std::_Exit(EXIT_FAILURE);
+    }
+    if (n == "nonzeroexit_stderroutput") {
+        std::cerr << "err" << std::endl;
+        std::_Exit(EXIT_FAILURE);
+    }
+    if (n == "invalid_json") {
+        std::cout << "{" << std::endl;
+        std::_Exit(EXIT_SUCCESS);
+    }
+    if (n == "pass_stdin_to_stdout") {
+        std::string s;
+        std::getline(std::cin, s);
+        std::cout << s << std::endl;
+        std::_Exit(EXIT_SUCCESS);
+    }
+    std::cerr << "Unknown mock process: " << n << std::endl;
+    std::_Exit(EXIT_FAILURE);
+}()};
+} // namespace
+
 BOOST_FIXTURE_TEST_SUITE(system_tests, BasicTestingSetup)
 
 #ifdef ENABLE_EXTERNAL_SIGNER
 
-static std::vector<std::string> mock_executable(std::string name)
+static std::vector<std::string> mock_executable(const std::string& name)
 {
-    // Invoke the mock_process/* test case with all unsolicited output suppressed.
-    return {
-        boost::unit_test::framework::master_test_suite().argv[0],
-        // Disable false-positive memory leak dumps to stderr
-        // in debug builds when using the Windows UCRT.
-        "--detect_memory_leaks=0",
-        // Disable logging to stdout.
-        "--log_level=nothing",
-        // Disable the test report to stderr.
-        "--report_level=no",
-        "--run_test=mock_process/" + name,
-    };
+#if defined(WIN32)
+    _putenv_s(MOCK_PROCESS_ENV, name.c_str());
+#else
+    setenv(MOCK_PROCESS_ENV, name.c_str(), /*overwrite=*/1);
+#endif
+    return {boost::unit_test::framework::master_test_suite().argv[0]};
 }
 
 BOOST_AUTO_TEST_CASE(run_command)
@@ -67,7 +99,7 @@ BOOST_AUTO_TEST_CASE(run_command)
         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 %d: \n", util::Join(command, " "), boost::exit_test_failure)) != std::string::npos);
+            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned %d: \n", util::Join(command, " "), EXIT_FAILURE)) != std::string::npos);
             return true;
         });
     }
@@ -77,7 +109,7 @@ BOOST_AUTO_TEST_CASE(run_command)
         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, " "), boost::exit_test_failure, "err")) != std::string::npos);
+            BOOST_CHECK(what.find(strprintf("RunCommandParseJSON error: process(%s) returned %s: %s", util::Join(command, " "), EXIT_FAILURE, "err")) != std::string::npos);
             BOOST_CHECK(what.find(expected) != std::string::npos);
             return true;
         });
diff --git a/test/lint/lint-includes.py b/test/lint/lint-includes.py
index 18f47240..2e8417ea 100755
--- a/test/lint/lint-includes.py
+++ b/test/lint/lint-includes.py
@@ -21,7 +21,6 @@ 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",