remotefilelog: have command wait for background task in the tests

We leverage the feature introduced in the two previous commit to have the main
command wait for the background task to be finished before returning.

All test still pass and instability seems to have gone away.

Most of the sleep and explicit wait are now useless and will be cleaned up in
later changesets.

Differential Revision: https://phab.mercurial-scm.org/D7586

tests/test-remotefilelog-bgprefetch.t

@@ -6,6 +6,7 b''
   $ cat >> $HGRCPATH <<EOF
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master

tests/test-remotefilelog-repack-fast.t

@@ -8,6 +8,7 b''
   > fastdatapack=True
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master

tests/test-remotefilelog-repack.t

@@ -6,6 +6,7 b''
   $ cat >> $HGRCPATH <<EOF
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master