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.