Fix HighAvailabilityManagerSplitBrainTest

Test was failing non-deterministically in various branch jenkins builds.

- If testStandbyRebind ran before testDoubleRebindFails, then the
  latter would fail because TestEntityFailingRebind.throwOnRebind
  had been set to false.
- Therefore always restore the value in tearDown, and set it to
  true in setUp