generic/050: fix _require_local_device $SCRATCH_DEV check order

generic/050: fix _require_local_device $SCRATCH_DEV check order

Commit 2b4eae7fd825 ("common/rc: add scratch shutdown support for
overlayfs") added a _require_local_device check to generic tests 042
and 050.  However, for test 050, this check was added _before_
actually verifying that a SCRATCH_DEV actually exists.  This patch
simply re-orders the _require_local_device to the right place.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>