Committed with a couple of changes from the posted patch: commented on the multiple dirname() calls as Serhiy suggested, removed the 'with DirsOnSysPath():' line since it turns out to be unnecessary when supplying top_level_dir to loader.discover(), and added some documentation for the new function to Doc/library/test.rst.

Victor, I went ahead and did the change to test_asyncio since it looks like the tulip project test package doesn't actually have __init__.py or __main__.py, and thus should be no conflict.  Please correct me if I'm wrong on that.

Thanks for the reviews!