btrfs/301: enhance nested simple quotas test

btrfs/301: enhance nested simple quotas test

We had a bug with qgroup accounting with 2+ layers, which was most
easily detected with a slightly more complex nested squota hierarchy.
Make the nested squota test case a tiny bit more complex to capture this
test as well.

The kernel patch that this change exercises is:
        btrfs: fix iteration bug in __qgroup_excl_accounting()

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>