xfs/realtime: Add require_no_realtime function

xfs/realtime: Add require_no_realtime function

Some tests do not play well with realtime devices, in an effort to
produce a stable set of test which exercise the realtime code paths
we introduce a _require_no_realtime function to allow tests to opt
out of realtime subvolume test runs.

And to make tests generic/409-411 work well with rt device, teach
_get_mount now honors $SCRATCH_OPTIONS.

Signed-off-by: Richard Wareing <rwareing@fb.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>