SPARK-2932 [STREAMING] Move MasterFailureTest out of "main" source directory

(HT @vanzin) Whatever the reason was for having this test class in `main`, if there is one, appear to be moot. This may have been a result of earlier streaming test reorganization.

This simply puts `MasterFailureTest` back under `test/`, removes some redundant copied code, and touches up a few tiny inspection warnings along the way.