Refs #36652, #36936 -- Improved path manipulation in a migration test launching a subprocess.

Author: pmppk

tests/migrations/test_loader.py

@@ -698,7 +698,9 @@ def test_loading_order_does_not_create_circular_dependency(self):
             test_settings.write(f"INSTALLED_APPS += {INSTALLED_APPS}\n")
 
         test_environ = os.environ.copy()
-        test_environ["PYTHONPATH"] = str(tests_dir)
+        test_python_path = sys.path.copy()
+        test_python_path.append(str(tests_dir))
+        test_environ["PYTHONPATH"] = os.pathsep.join(test_python_path)
         # Ensure deterministic failures.
         test_environ["PYTHONHASHSEED"] = "1"