fstests: check if the scratch device is an lv device for certain tests

fstests: check if the scratch device is an lv device for certain tests

I use lvm to carve up a large disk so I can run the btrfs raid related xfstests.
However this messes with tests that try to greate lvm devices ontop of
SCRATCH_DEV.  Handle this by adding a _require_scratch_nolvm helper to skip
tests that are going to try and create lvm devices.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>