fix prospective-parachains best backable chain reversion bug (#6417)
Kudos to @EclesioMeloJunior for noticing it Also added a regression test for it. The existing unit test was exercising only the case where the full chain is reverted --------- Co-authored-by: GitHub Action <[email protected]> Co-authored-by: Bastian Köcher <[email protected]> (cherry picked from commit 05ad5475)