btrfs: Add new 'limit' test group for btrfs

btrfs: Add new 'limit' test group for btrfs

Btrfs qgroup also supports to limit the usage of specified qgroups.

It's possible to enable qgroup but doesn't enable limit.
(Most user won't use qgroup limit for various problems)

So add a new test group 'limit' for btrfs, as a subset of existing
'qgroup' group.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>