Add a test file to importlib that runs regrtest using importlib.__import__.

Add a test file to importlib that runs regrtest using importlib.__import__.

The file must be run using runpy. Certain tests are currently excluded from
being run as they have known failures based on golden value checks that fail
for various reasons (typically because __loader__ is not expected to be set on
modules). Running the tests with this file does discover some incompatibilites
in importlib that will be fixed in the near future (as noted currently in the
docstring).