[SPARK-11466][CORE] Avoid mockito in multi-threaded FsHistoryProviderSuite test.

The test functionality should be the same, but without using mockito; logs don't
really say anything useful but I suspect it may be the cause of the flakiness,
since updating mocks when multiple threads may be using it doesn't work very
well. It also allows some other cleanup (= less test code in FsHistoryProvider).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9425 from vanzin/SPARK-11466.