fix convert tests when binary has spaces, or is absolute path on Mac
fix convert tests when binary has spaces, or is absolute path on Mac

Summary: A port of https://github.com/facebook/flow/pull/1124 to the convert test runner

Reviewed By: samwgoldman

Differential Revision: D2976357

fb-gh-sync-id: 20430b21d836dd376dac5867f873018bdcc8ce9a
shipit-source-id: 20430b21d836dd376dac5867f873018bdcc8ce9a

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