fstests: add fiemap group

fstests: add fiemap group

Add a fiemap group for all tests that exercise fiemap, either directly
through xfs_io, or more indirectly like through filefrag or helpers in
common/* (like _count_extents, _count_holes, etc). This is useful in
order to quickly test changes made to the fiemap implementation of a
filesystem for example.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>