don't allow Windows test failures
don't allow Windows test failures

Summary:
our tests used to be so flaky on Windows that we would allow up to 20% of them to randomly fail and still consider it a success.

I expect them to be stable now, so I'm removing that hack.

Reviewed By: gkz

Differential Revision: D24899577

fbshipit-source-id: b912cbd4915418f0933ffa1bba7f2d326ddb12a8

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