btrfs: tests: add a test for btrfs_add_extent_mapping

This helper is different from the normal add_extent_mapping in that it
will stuff an em into a gap that exists between overlapping em's in the
tree.  It appeared there was a bug so I wrote a self test to validate it
did the correct thing when it worked with two side by side ems.
Thankfully it is correct, but more testing is better.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>