test: stabilize test-remotefilelog-bgprefetch.t flaky output When running the test suite with multiple processes, we often get flaky outputs, like here: https://ci.octobus.net/job/MercurialPy2/274/console ``` - $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histidx - $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histpack - $TESTTMP/hgcache/master/packs/f4d50848e0b465e9bfd2875f213044c06cfd7407.dataidx - $TESTTMP/hgcache/master/packs/f4d50848e0b465e9bfd2875f213044c06cfd7407.datapack + $TESTTMP/hgcache/master/11/f6ad8ec52a2984abaafd7c3b516503785c2072/1406e74118627694268417491f018a4a883152f0 + $TESTTMP/hgcache/master/11/f6ad8ec52a2984abaafd7c3b516503785c2072/ef95c5376f34698742fe34f315fd82136f8f68c0 + $TESTTMP/hgcache/master/39/5df8f7c51f007019cb30201c49e884b46b92fa/69a1b67522704ec122181c0890bd16e9d3e7516a + $TESTTMP/hgcache/master/95/cb0bfd2977c761298d9624e4b4d4c72a39974a/076f5e2225b3ff0400b98c92aa6cdf403ee24cca + $TESTTMP/hgcache/master/af/f024fe4ab0fece4091de044c58c9ae4233383a/bb6ccd5dceaa5e9dc220e0dad65e051b94f69a2c ``` Add some sleeps after `debugwaitonrepack` calls as it seems it needs some extra time to cleanup. Differential Revision: https://phab.mercurial-scm.org/D5960