Switch to using Plutus testingCostModels instead of zero models in Tests

[teodanciu]

Because of an issue in testingCostModels from Plutus, we had to set our costmodels to zero values in order to get the tests to pass for the CIP-0069 PR.

Once the problem is solved in Plutus, we have to revert to the cost models they provide, for more realistic tests.

[lehins]

Is there a relevant issue on the Plutus repo that we can cross reference?