generic: add _require_block_device to more tests

generic: add _require_block_device to more tests

generic/067 mounts $SCRATCH_DEV directly in the test, assuming it's
a block device. generic/299 and generic/300 query the size of
$SCRATCH_DEV by running 'blockdev --getsz $SCRATCH_DEV'.

So add the check to make sure $SCRATCH_DEV is a real block device in
these tests.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>