test: follow-up on 1c4d6ab2ecb8, stabilize test-remotefilelog-bgprefetch.t

In 1c4d6ab2ecb8, I've added some sleeps after about half of the calls to `hg
debugwaitonrepack` but it seems the other calls are also flaky as catched by
our CI: https://ci.octobus.net/job/MercurialPy2/299/console. Add sleeps after
all calls in order to be safe.

tests/test-remotefilelog-bgprefetch.t

@@ -142,6 +142,7 b''
   $ hg debugwaitonprefetch >/dev/null 2>%1
   $ sleep 1
   $ hg debugwaitonrepack >/dev/null 2>%1
+  $ sleep 1
   $ find $CACHEDIR -type f | sort
   $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histidx
   $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histpack
@@ -287,6 +288,7 b''
 
   $ sleep 0.5
   $ hg debugwaitonrepack >/dev/null 2>%1
+  $ sleep 0.5
 
   $ find $CACHEDIR -type f | sort
   $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histidx
@@ -331,6 +333,7 b''
   * files fetched over 1 fetches - (* misses, 0.00% hit ratio) over *s (glob) (?)
   $ sleep 0.5
   $ hg debugwaitonrepack >/dev/null 2>%1
+  $ sleep 0.5
 
   $ find $CACHEDIR -type f | sort
   $TESTTMP/hgcache/master/packs/8f1443d44e57fec96f72fb2412e01d2818767ef2.histidx