test: Fail on self-check warnings in test_runner.py

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 97cbbeb2..285f9251 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -326,7 +326,7 @@ jobs:
           BITCOINWALLET: '${{ github.workspace }}\build\bin\Release\bitcoin-wallet.exe'
           BITCOINCHAINSTATE: '${{ github.workspace }}\build\bin\Release\bitcoin-chainstate.exe'
           TEST_RUNNER_EXTRA: ${{ github.event_name != 'pull_request' && '--extended' || '' }}
-        run: py -3 test/functional/test_runner.py --jobs $NUMBER_OF_PROCESSORS --ci --quiet --tmpdirprefix="${RUNNER_TEMP}" --combinedlogslen=99999999 --timeout-factor=${TEST_RUNNER_TIMEOUT_FACTOR} ${TEST_RUNNER_EXTRA}
+        run: py -3 test/functional/test_runner.py --jobs $NUMBER_OF_PROCESSORS --quiet --tmpdirprefix="${RUNNER_TEMP}" --combinedlogslen=99999999 --timeout-factor=${TEST_RUNNER_TIMEOUT_FACTOR} ${TEST_RUNNER_EXTRA}
 
       - name: Clone corpora
         if: matrix.job-type == 'fuzz'
@@ -501,7 +501,7 @@ jobs:
         env:
           TEST_RUNNER_EXTRA: ${{ github.event_name != 'pull_request' && '--extended' || '' }}
         run: |
-          py -3 test/functional/test_runner.py --jobs $NUMBER_OF_PROCESSORS --ci --quiet --tmpdirprefix="$RUNNER_TEMP" --combinedlogslen=99999999 --timeout-factor=$TEST_RUNNER_TIMEOUT_FACTOR $TEST_RUNNER_EXTRA \
+          py -3 test/functional/test_runner.py --jobs $NUMBER_OF_PROCESSORS --quiet --tmpdirprefix="$RUNNER_TEMP" --combinedlogslen=99999999 --timeout-factor=$TEST_RUNNER_TIMEOUT_FACTOR $TEST_RUNNER_EXTRA \
             `# feature_unsupported_utxo_db.py fails on Windows because of emojis in the test data directory.` \
             --exclude feature_unsupported_utxo_db.py \
             `# See https://github.com/bitcoin/bitcoin/issues/31409.` \
diff --git a/ci/test/03_test_script.sh b/ci/test/03_test_script.sh
index 1cf9eb23..1bd9fcfc 100755
--- a/ci/test/03_test_script.sh
+++ b/ci/test/03_test_script.sh
@@ -184,7 +184,7 @@ if [ "$RUN_FUNCTIONAL_TESTS" = "true" ]; then
   eval "TEST_RUNNER_EXTRA=($TEST_RUNNER_EXTRA)"
   LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" \
   "${BASE_BUILD_DIR}/test/functional/test_runner.py" \
-    --ci "${MAKEJOBS}" \
+    "${MAKEJOBS}" \
     --tmpdirprefix "${BASE_SCRATCH_DIR}/test_runner/" \
     --ansi \
     --combinedlogslen=99999999 \
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 425205b3..ae2320a1 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -410,7 +410,6 @@ def main():
     parser.add_argument('--ansi', action='store_true', default=sys.stdout.isatty(), help="Use ANSI colors and dots in output (enabled by default when standard output is a TTY)")
     parser.add_argument('--combinedlogslen', '-c', type=int, default=0, metavar='n', help='On failure, print a log (of length n lines) to the console, combined from the test framework and all test nodes.')
     parser.add_argument('--coverage', action='store_true', help='generate a basic coverage report for the RPC interface')
-    parser.add_argument('--ci', action='store_true', help='Run checks and code that are usually only enabled in a continuous integration environment')
     parser.add_argument('--exclude', '-x', action='append', help='specify a script to exclude. Can be specified multiple times. The .py extension is optional.')
     parser.add_argument('--extended', action='store_true', help='run the extended test suite in addition to the basic tests')
     parser.add_argument('--help', '-h', '-?', action='store_true', help='print help text and exit')
@@ -425,7 +424,8 @@ def main():
     parser.add_argument('--resultsfile', '-r', help='store test results (as CSV) to the provided file')
 
     args, unknown_args = parser.parse_known_args()
-    fail_on_warn = args.ci
+    # Fail on self-check warnings before running the tests.
+    fail_on_warn = True
     if not args.ansi:
         global DEFAULT, BOLD, GREEN, RED
         DEFAULT = ("", "")