Fix incomplete sorting. (#4795)

* Fix incomplete sorting.

* fmt.

* Better test.

* Update runtime/parachains/src/disputes.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* chore fmt

* simplify the sorting for two items

* add test for assure duplicates are detected

* fixup tests

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>