test_importlib fails when _testcapi is a builtin module

[tiran]

Bug report

Some test_importlib cases for extension modules are failing when Python is compiled with dynamic extension support but _testcapi or _testmultiphase are builtin modules:

======================================================================
FAIL: test_module (test.test_importlib.extension.test_finder.Frozen_FinderTests.test_module)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/python-wasm/cpython/Lib/test/test_importlib/extension/test_finder.py", line 25, in test_module
 self.assertTrue(self.find_spec(util.EXTENSIONS.name))
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: None is not true

The tests should be skipped if the module name is in sys.builtin_module_names.

[tiran]

https://buildbot.python.org/all/#/builders/1056/builds/1/steps/10/logs/stdio