remotefilelog: have command wait for background task in the tests stable

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

--- a/tests/test-remotefilelog-bgprefetch.t	Mon Dec 09 09:54:27 2019 +0100
+++ b/tests/test-remotefilelog-bgprefetch.t	Mon Dec 09 09:57:34 2019 +0100
@@ -6,6 +6,7 @@
   $ cat >> $HGRCPATH <<EOF
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master

--- a/tests/test-remotefilelog-repack-fast.t	Mon Dec 09 09:54:27 2019 +0100
+++ b/tests/test-remotefilelog-repack-fast.t	Mon Dec 09 09:57:34 2019 +0100
@@ -8,6 +8,7 @@
   > fastdatapack=True
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master

--- a/tests/test-remotefilelog-repack.t	Mon Dec 09 09:54:27 2019 +0100
+++ b/tests/test-remotefilelog-repack.t	Mon Dec 09 09:57:34 2019 +0100
@@ -6,6 +6,7 @@
   $ cat >> $HGRCPATH <<EOF
   > [devel]
   > remotefilelog.ensurestart=True
+  > remotefilelog.bg-wait=True
   > EOF
 
   $ hg init master