btrfs: add test for btrfstune squota enable/disable

btrfs: add test for btrfstune squota enable/disable

btrfstune supports enabling simple quotas on a fleshed out filesystem
(without adding owner refs) and clearing squotas entirely from a
filesystem that ran under squotas (clearing the incompat bit)

Test that these operations work on a relatively complicated filesystem
populated by fsstress while preserving fssum.

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>