Fix and re-instate Babbage `simpleRewards` test
`allPoolStakeCert` needed some adjustments after the `UMap` purge in `cardano-ledger` and had to make sure that we don't re-register already registered staking credentials. Also, because validations were turned off apparently, `FailedToValidateSlot` was not triggered but rather we ended up in an invalid state (the associated delegations were dropped in the account state), thus resulting in a test failure. With validation, attempting to re-register already registered staking credentials would trigger the `StakeKeyAlreadyRegisteredDELEG` predicate failure as per the `DELEG` rule.