Support Bazel's TEST_PREMATURE_EXIT_FILE and add it to CLI as well

This tells Catch2 to create an empty file at specified path before
the tests start, and delete it after the tests finish. This allows
callers to catch cases where the test binary silently exits before
finishing (e.g. via call to `exit(0)` inside the code under test),
by looking whether the file still exists.

Closes #3020

tests/TestScripts/testBazelExitGuardFile.py

@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+
+#              Copyright Catch2 Authors
+# Distributed under the Boost Software License, Version 1.0.
+#   (See accompanying file LICENSE.txt or copy at
+#        https://www.boost.org/LICENSE_1_0.txt)
+
+# SPDX-License-Identifier: BSL-1.0
+
+import os
+import sys
+import subprocess
+
+def generate_path_suffix() -> str:
+    return os.urandom(16).hex()[:16]
+
+
+def run_common(cmd, env = None):
+    cmd_env = env if env is not None else os.environ.copy()
+    print('Running:', cmd)
+
+    try:
+        ret = subprocess.run(
+            cmd,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            check=True,
+            universal_newlines=True,
+            env=cmd_env
+        )
+    except subprocess.SubprocessError as ex:
+        print('Could not run "{}"'.format(cmd))
+        print("Return code: {}".format(ex.returncode))
+        print("stdout: {}".format(ex.stdout))
+        print("stderr: {}".format(ex.stderr))
+        raise
+
+
+def test_bazel_env_vars(bin_path, guard_path):
+    env = os.environ.copy()
+    env["TEST_PREMATURE_EXIT_FILE"] = guard_path
+    env["BAZEL_TEST"] = '1'
+    run_common([bin_path], env)
+
+def test_cli_parameter(bin_path, guard_path):
+    cmd = [
+      bin_path,
+      '--premature-exit-guard-file',
+      guard_path
+    ]
+    run_common(cmd)
+
+def test_no_crash(bin_path, guard_path):
+    cmd = [
+      bin_path,
+      '--premature-exit-guard-file',
+      guard_path,
+      # Disable the quick-exit test
+      '~[quick_exit]'
+    ]
+    run_common(cmd)
+
+checks = {
+  'bazel': (test_bazel_env_vars, True),
+  'cli': (test_cli_parameter, True),
+  'no-crash': (test_no_crash, False),
+}
+
+
+bin_path = os.path.abspath(sys.argv[1])
+output_dir = os.path.abspath(sys.argv[2])
+test_kind = sys.argv[3]
+guard_file_path = os.path.join(output_dir, f"guard_file.{generate_path_suffix()}")
+print(f'Guard file path: "{guard_file_path}"')
+
+check_func, file_should_exist = checks[test_kind]
+check_func(bin_path, guard_file_path)
+
+assert os.path.exists(guard_file_path) == file_should_exist
+# With randomly generated file suffix, we should not run into name conflicts.
+# However, we try to cleanup anyway, to avoid having infinity files in
+# long living build directories.
+if file_should_exist:
+    try:
+        os.remove(guard_file_path)
+    except Exception as ex:
+        print(f'Could not remove file {guard_file_path} because: {ex}')
+