Fix test assertion

Author: hasier

mypy/test/test_config_parser.py

@@ -174,7 +174,7 @@ def set_strict_flags() -> None:
                 assert strict_option_set is True
                 assert options.ignore_missing_imports_per_module is True
                 assert options.config_file == str(pyproject.name)
-                os.environ["MYPY_CONFIG_FILE_DIR"] = str(pyproject.parent)
+                assert os.environ["MYPY_CONFIG_FILE_DIR"] == str(pyproject.parent)
 
                 assert options.per_module_options["c"] == {
                     "disable_error_code": [],