Randomly restart initial sync in the dynamic initial sync passthrough

Currently we randomly restart initial sync during initial sync in the dynamic resync passthrough: https://github.com/mongodb/mongo/blob/r3.4.0-rc1/buildscripts/resmokelib/testing/hooks.py#L261

We should remove the self.use_resync check so that we add coverage for shutting down initial sync during initial sync. Given the bugs we've had at shutdown recently, extra test coverage there would be good.