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Right now to run importlib tests you can do either of the following:
./python -m tests test_importlib
./python -m tests.test_importlib
Both make use of the regrtest infrastructure. For test submodules the commands are similar:
./python -m tests test_importlib.test_api
./python -m tests.test_importlib.test_api
You can also use unittest directly when testing specific test modules:
./python -m unittest tests.test_importlib.test_api
./python -m unittest tests.test_importlib.test_api.Source_ReloadTests.test_reload_location_changed
However, currently you cannot use unittest directly with a test package:
./python -m unittest tests.test_importlib
./python -m unittest tests.test_importlib.source
It would be nice to be able to do so, rather than switching back and forth between the unittest CLI and the regrtest CLI.
The change to do so is relatively straight-forward using the "load_tests" protocol*. Just add the following to the __init__.py of the test packages:
 def load_tests(loader, tests, pattern):
 from test import TEST_HOME_DIR as topdir
 startdir = os.path.dirname(__name__)
 pkgtests = loader.discover(startdir, pattern or 'test*.py', topdir)
 tests.addTests(pkgtests)
 return tests
The boilerplate could even be moved to tests.support as a factory function:
 def make_load_tests(modfilename):
 from test import TEST_HOME_DIR as topdir
 startdir = os.path.dirname(modfilename)
 def load_tests(loader, tests, pattern):
 pkgtests = loader.discover(startdir, pattern or 'test*.py', topdir)
 tests.addTests(pkgtests)
 return tests
 return load_tests
In the test package __init__.py:
 load_tests = support.make_load_tests(__name__)
Then using unittest directly with an importlib test package will work as expected. This is also something that could readily apply to any other test packages in the suite, which is why the factory function in test.support makes sense.
* https://docs.python.org/2/library/unittest.html#load-tests-protocol 

Personally I think the fact that this doesn't work by default is a bug in unittest. See issue 15007. Issue 16662 may also be related.

This has been covered by #22002.