StreamTracer should be mockable

#3305 put DoNotMock on StreamTracer saying that the mocks are not thread-safe. But [mockito claims they are](https://github.com/mockito/mockito/wiki/FAQ#is-mockito-thread-safe), except for stubbing and verify methods. We need to figure out where the discrepancy is and either file a bug against Mockito or let the StreamTracer be mocked.

I discovered this as part of removing DoNotMock for #4452. For my changes, I left things as-is, but I found the documentation I wrote to be questionable.

CC @carl-mastrangelo