remotefilelog: reduce probability of race-condition in remotefilelog tests ca1014ad3de4 introduced a new parameter `ensurestart` to speed up remotefilelog background processes start. Unfortunately it seems to have increased the possibility of race-conditions in remotefilelog tests testing those background processes. With `ensurestart=False`, it seems that it's more probable to enter in a race condition with `debugwaitonprefetch` and `debugwaitonrepack` in remotefilelog background tests. Our CI seems to have a high probability of triggering this race condition so make it configurable to ensure tests are stable. Differential Revision: https://phab.mercurial-scm.org/D6772