Stress is a test helper that puts stress on a service's start and stop
functions so that we can detect any data races that it might have due to
improper use of BaseStopStart.
StressErr is the same as Stress except that the given allowedStartErr is
tolerated on start (either no error or an error that is allowedStartErr is
allowed). This is useful for services that may want to return an error if
they're shut down as they're still starting up.