Improve fee and script budget calculation in ImpTest fixup

[teodanciu]

The script budget (set in the redeemers) depends on the value of the fee, which in turn depends on the value of the budget.

The current solution (implemented in #4766) is to first compute the fee, maximizing it with respect to the budget (so assuming the maximum budget), then computing the real budget, with a previously computed more realistic fee. Unfortunately this is not precisely enough in some cases, so it also adds a 10% to the fee as a margin - because in some cases the required fee would still go over.

A more elegant and reliable way to satisfy this circular dependency between redeemers and fee would be to find a fixed point by calling fixup functionality until the transaction stops changing. This would require some refactoring of the fixup (one idea: making the fixup idempotent).