iotests: add test 257 for bitmap-mode backups

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190709232550.10724-18-jsnow@redhat.com
[Removed 'auto' group, as per new testing config guidelines --js]
Signed-off-by: John Snow <jsnow@redhat.com>
3 files changed