DirectoryWatcher: also wait for the notifier thread

Ultimately the DirectoryWatcher is not ready until the notifier thread
is also active.  Failure to wait for the notifier thread may result in
loss of events.  While this is not catastrophic in practice, the tests
are sensitive to this as depending on the thread scheduler, the thread
may fail to being execution before the operations are completed by the
fixture.  Running this in a tight loop shows no regressions locally as
previously, but this failure mode was been sighted once on a builder.