btrfs: add test for incremental send for file with shared extents

btrfs: add test for incremental send for file with shared extents

Test that an incremental send operation works correctly when a file has
shared extents with itself in the send snapshot, with a hole between them,
and the file size has increased in the send snapshot.

This currently fails in 5.5-rc kernels (regression) but is fixed by a
patch that has the following subject:

  Btrfs: send, fix emission of invalid clone operations within the same file

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>