Fix occasional ExUnitsTooBigUTxO failures in alonzo-test

[neilmayhew]

Reproducible with:

$ cabal test cardano-ledger-alonzo-test --test-options='
  --pattern="/Ada is preserved/"
  --quickcheck-replay="(SMGen 17491612495563005545 16350810121916512025,83)"
'
...
Exception:
  LEDGER sigGen: UtxowFailure (ShelleyInAlonzoUtxowPredFailure (UtxoFailure (ExUnitsTooBigUTxO (WrapExUnits {unWrapExUnits = ExUnits' {exUnitsMem' = 537375, exUnitsSteps' = 1891529}}) (WrapExUnits {unWrapExUnits = ExUnits' {exUnitsMem' = 550000, exUnitsSteps' = 1099989}})))) :| []

It would be helpful to turn this into a discard, if that's possible. It may be possible to find some prior art for dry-running scripts in the Imp tests. Alternatively, we could turn all predicate failures into discards using try or onException.