test: isolate CI summary and output files

tests/test_ci_output_isolation.py

@@ -0,0 +1,37 @@
+"""Run the real unit tests with external CI files and detect leaked writes."""
+
+import os
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+from ci_test_case import IsolatedCITestCase
+
+
+class CIOutputIsolationTests(IsolatedCITestCase):
+    def test_unit_suite_preserves_external_ci_files(self):
+        tests = Path(__file__).resolve().parent
+        modules = sorted(p.stem for p in tests.glob('test_*.py') if p.name != Path(__file__).name)
+        with tempfile.TemporaryDirectory() as scratch:
+            environment = os.environ.copy()
+            environment['PYTHONPATH'] = str(tests) + os.pathsep + environment.get('PYTHONPATH', '')
+            expected = {}
+            for variable in ('GITHUB_STEP_SUMMARY', 'GITHUB_OUTPUT'):
+                path = Path(scratch) / variable
+                content = f'external {variable}\n'
+                path.write_text(content)
+                environment[variable] = str(path)
+                expected[path] = content
+            result = subprocess.run(  # noqa: S603 -- Run local test modules with the current Python interpreter.
+                [sys.executable, '-m', 'unittest', *modules, '-q'],
+                cwd=tests.parent,
+                env=environment,
+                capture_output=True,
+                text=True,
+                check=False,
+                timeout=60,
+            )
+            self.assertEqual(result.returncode, 0, result.stdout + result.stderr)
+            for path, content in expected.items():
+                self.assertEqual(path.read_text(), content, f'Unit tests wrote to external {path.name}')