Fixed test issuing non-serialized messages to the subject.

Should resolve the test failure of #1972 .

When the main part of the test succeded, an onCompleted was sent out from the main thread which was not serialized in respect to the onNext(1) issued from the worker thread, therefore, two replay was attempted sometimes. At first the caughtUp was seen false, the replay was entered but the next instruction cleared the index causing the IllegalStateException.