test: fix cwd_and_chdir test on Unix

It was broken since for ever, but e8c4c329e69fff1b656c833883997ac663ea48ef added the assert which made it blow up on Unix.

R=@piscisaureus
@piscisaureus: Yes, that's what that code is supposed to do, but it doesn't and your assert proved it :-)

@bnoordhuis: I guess it should be size1> 1, right? To avoid the case when the path is "/".