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
Two things:

* uv_cwd() should really be consistent in it's behavior. On windows it removes the trailing slash from a path, unless it's the root. I guess unix should do the same.
Of course we can't do that until v1.1

* The code is kind of unclear. What about:
`if (size > 1 && buf_orig[size1 - 1] == '/')`
... and get rid of the strrchr() call.