btrfs: tests: test invalid splitting when skipping pinned drop extent_map

This reproduces the bug fixed by "btrfs: fix incorrect splitting in
btrfs_drop_extent_map_range", we were improperly calculating the range
for the split extent.  Add a test that exercises this scenario and
validates that we get the correct resulting extent_maps in our tree.

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