testsuite: Add and document a 'should' for "idle" -> "recover" RAID test

testsuite: Add and document a 'should' for "idle" -> "recover" RAID test

When a "recover" is just starting for a RAID LV, it is possible to get
"idle" for the sync action if the status is issued quickly enough.  This
is fine, the MD thread just hasn't gotten things going yet.  However,
the /need/ for a "recover" should be marked in md->recovery and it would
be simple enough to fix the kernel so this doesn't happen.  May eventually
want a separate bug for this, but for now it fits with RHBZ 1507719.