Move test_zipfile to unittest.main()
Spruce up test_xmlrpc by using ModuleNotFoundError and moving to
unittest.main().
Move test___all__ over to unittest.main() and use ModuleNotFoundError
Partially revert changeset #281857369a78 to make sure threads are
reaped in all situations.
Update various test modules to use unittest.main() for test discovery
instead of manually listing tests for test.support.run_unittest().
Issue #15767: Touch up ModuleNotFoundError usage by import.

Forgot to raise ModuleNotFoundError when None is found in sys.modules.
This led to introducing the C function PyErr_SetImportErrorSubclass()
to make setting ModuleNotFoundError easier.

Also updated the reference docs to mention ModuleNotFoundError
appropriately. Updated the docs for ModuleNotFoundError to mention the
None in sys.modules case.

Lastly, it was noticed that PyErr_SetImportError() was not setting an
exception when returning None in one case. That issue is now fixed.
Issue #15767: Add an explicit test for raising ModuleNotFoundError
when None in sys.modules.
Issue #18048: Rename test_pep263.py to test_source_encoding.py.
Issue #18048: Rename test_coding.py to test_source_encoding.py.
Issue #18048: Merge test_pep263.py and test_coding.py into test_source_encoding.py.