Implementing an idea from Guido on the checkins list:

Implementing an idea from Guido on the checkins list:
When regrtest.py finds an attribute "test_main" in a test it imports,
regrtest runs the test's test_main after the import.  test_threaded_import
needs this else the cross-thread import lock prevents it from making
progress.  Other tests can use this hack too, but I doubt it will ever be
popular.