fstests: add a another gap extent testcase for btrfs

fstests: add a another gap extent testcase for btrfs

This is a testcase for a corner that I missed when trying to fix gap
extents for btrfs.  We would end up with gaps if we hole punched past
isize and then extended past the gap in a specific way.  This is a
simple reproducer to show the problem, and has been properly fixed by my
patches now.

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