generic/{628,629}: fix order of _require_scratch* and _require_dm_target

generic/{628,629}: fix order of _require_scratch* and _require_dm_target

Test cases with dm_target shall call _require_scratch* before
_require_dm_target to ensure that valid SCRATCH_DEV is available for
dm_target. However, the test cases generic/628 and generic/629 call
_require_dm_target before _require_scratch*, then unexpected failure
was reported when SCRATCH_DEV is not specified. Fix the order of
function calls.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>