Wait for all threads to finish executing before leaving in an EventBase test

Wait for all threads to finish executing before leaving in an EventBase test

Summary:
If one of the assertions failed, it would result in any threads that are still running accessing already-disposed data, so wait for the threads to exit, even when an exception is thrown.

This also slightly cleans up the handling of threads a bit further down, because the mess it was doing was completely unnecessary.

Reviewed By: yfeldblum

Differential Revision: D3692438

fbshipit-source-id: 9082ba248b2cf1062e46c97faf0bc062312ee9ae