The attached patch makes 'test_main' optional for stdlib tests.  If a test module does not have a 'test_main', regrtest will use the unittest loadTestsFromModule loader to load the tests.

This moves us further in the direction of using normal unittest facilities instead of specialized regrtest ones.  Any test module that can be correctly run currently using python unittest -m test.test_xxx could be converted to use this by simply deleting its test_main, thus no longer requiring manual maintenance of the list of tests to run.  Not all tests can be converted that easily, however, since test_main sometimes does some additional things (such as reap_children or reap_threads).