allows for __tests__ & __test__ directories

[cmcculloh-kr]

fixes #4435

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[rmunn]

I'd suggest also allowing __test__ in addition to __tests__, because I've seen both test and tests being used as directory names in projects I've been involved with. So while I haven't personally seen either __test__ or __tests__ myself, I wouldn't be surprised if both are in use, with more or less equal usage (I'd guess). So if we're special-casing one, we should special-case both and do it in the same PR so we don't have to revisit the question later on when someone asks for __test__.