btrfs: introduce btrfs/volume group

btrfs: introduce btrfs/volume group

The btrfs/volume group represent a set of btrfs test-cases, which
shall intend to verify the relevant btrfs volume operations.

Under this new group all the existing btrfs/replace group would come
under, and also the device operations test cases which does not have
any group as of now. This group is helpful to verify the btrfs
volume related changes.

Run as
  ./check -g btrfs/volume

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>