btrfs: add a test for defrag of contiguous file extents

btrfs: add a test for defrag of contiguous file extents

Test that defrag merges adjacent extents that are contiguous.
This exercises a regression fixed by a patchset for the kernel that is
comprissed of the following patches:

  btrfs: fix extent map merging not happening for adjacent extents
  btrfs: fix defrag not merging contiguous extents due to merged extent maps

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>