ioPath reset `pathmgr` before each unit test
ioPath reset `pathmgr` before each unit test

Summary:
**Summary:** Clean up unit tests.

**Details:** Add `unitTest.setUp()` method that resets `_pathmgr.cwd` and `_pathmgr._async_handlers_used` since those are class variables and we maintain a single `PathManager` object across all unit tests.

Reviewed By: sujitoc

Differential Revision: D26177433

fbshipit-source-id: bc33e2a1098d72ad49a3128aad0d3102a104d2b9

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