Remove redundant functions in TestResult
Remove redundant functions in TestResult

Summary:
`BuckResult` already implements `is_success()` and `is_failure()`.
`TestResult` no longer needs these functions.

Reviewed By: bobyangyf

fbshipit-source-id: 36e2d5a8143338b65068cea4af6d2d6ad3858e47

JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck