btrfs: selftests: add a selftest for deleting two out of three extents

Add a selftest creating three extents and then deleting two out of the
three extents.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>