InitialSyncerResetsOptimesOnNewAttempt unit test should set optime before starting initial syncer

This unit test has a slight race condition. It calls _setMyLastOptime after calling InitialSyncer::startup(). InitialSyncer::startup() schedules _startInitialSyncAttempt to be run immediately, and if that function gets run before _setMyLastOptime is called in the unit test, the assertion will fail. This can be fixed by setting the optime before the InitialSyncer is started.