Use native parser in testcheck.py only if env variable TEST_NATIVE_PARSER is set

Example: `TEST_NATIVE_PARSER=1 pytest mypy/test/testheck.py`.

Author: JukkaL

mypy/test/testcheck.py

@@ -136,7 +136,7 @@ def run_case_once(
         options = parse_options(original_program_text, testcase, incremental_step)
         options.use_builtins_fixtures = True
         options.show_traceback = True
-        options.native_parser = True  # XXX remove
+        options.native_parser = bool(os.environ.get("TEST_NATIVE_PARSER"))
 
         if options.num_workers:
             options.fixed_format_cache = True