Fix failing unit test `testParentEdgeCases` under Windows

Summary: Noticed that this one was failing under Windows for the test cases where the root path (`/`) was supposed to be returned. Was returned Windows-style, made it return UNIX-style. All others work fine (return slashes as-is).

Test Plan:
`arc unit --everything` before and after this patch on Windows.

Will try out Ubuntu in near future.

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: aran, Korvin, vrana

Differential Revision: https://secure.phabricator.com/D5497