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
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors