This adds a simple test that ensures the busy bit works for push backups,
as well as doubling as bonus test for incremental backups that get interrupted
by EIO errors.

Recording bit tests are already handled sufficiently by 236.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Tested-by: NEric Blake <eblake@redhat.com>
Message-id: 20190223000614.13894-11-jsnow@redhat.com
Signed-off-by: NJohn Snow <jsnow@redhat.com>