generic/038: assign the test to trim group of tests

generic/038: assign the test to trim group of tests

The test runs fstrim utility to stress a filesystem, however it implies
that the filesystem supports ioctl(FITRIM), this is checked in runtime by
_require_batched_discard() in the test itself, however the assignment of
the test to trim group is missing.

The intention of the change is to get better association between tests
and test groups.

Signed-off-by: Vladimir Zapolskiy <vladimir@tuxera.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>