Adds `load_package_tests` function to test.support, uses it in test_asyncio,
test_email, test_json, test_tools, test_importlib and all test_importlib
sub-packages to implement test discovery.

to avoid reporting success on empty tests.

unittest.TestCase in prep of running tests under frozen and source
importlib.

Previously __path__ was set to [__name__], but that could lead to bad
results if someone managed to circumvent the frozen importer and
somehow ended up with a finder that thought __name__ was a legit
directory/location.

This should make the Linux distros happy as it is now easier to leave
importlib's tests out of their base Python distribution.