test: follow-up on 1c4d6ab2ecb8, stabilize test-remotefilelog-bgprefetch.t In 1c4d6ab2ecb8, I've added some sleeps after about half of the calls to `hg debugwaitonrepack` but it seems the other calls are also flaky as catched by our CI: https://ci.octobus.net/job/MercurialPy2/299/console. Add sleeps after all calls in order to be safe.