Add support for a --builtin argument to importlib.test to trigger running

import-specific tests with __import__ instead of importlib.

Lib/importlib/test/__main__.py

+import importlib
+from importlib.test.import_ import util
 import os.path
 from test.support import run_unittest
+import sys
 import unittest
 
 
@@ -7,6 +10,8 @@ def test_main():
     start_dir = os.path.dirname(__file__)
     top_dir = os.path.dirname(os.path.dirname(start_dir))
     test_loader = unittest.TestLoader()
+    if '--builtin' in sys.argv:
+        util.using___import__ = True
     run_unittest(test_loader.discover(start_dir, top_level_dir=top_dir))