btrfs: add test case for qgroup account on shared extents

btrfs: add test case for qgroup account on shared extents

This is a regression test of
'commit fcebe4562dec ("Btrfs: rework qgroup accounting")',
it's used to verify that removing shared extents can end up incorrect
qgroup accounting.

It can produce qgroup related warnings.

The fix is https://patchwork.kernel.org/patch/5499981/
"Btrfs: fix a warning of qgroup account on shared extents"

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Tested-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reviewed-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>