strip root from getDefCommand error case
strip root from getDefCommand error case

Summary: If a test case results in an error, we need to exclude the root from the filename so that the test's output is stable across test runs.

Reviewed By: mroch

Differential Revision: D22508461

fbshipit-source-id: 7b0949870164b378f3ab05edb611bdc07033a601

facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow