Fix multiple crypto unit test by not showing generated instances
There's an issue with the `deriving via` clause for OneEraHeader: * `show`ing a `NS Header (CardanoEras StandardCrypto BatchCompatibleCrypto)` works fine * `show`ing a `OneEraHeader (CardanoEras ...)` yields the annoying error message about not being able to handle 2 different cryptos Using forAllBlind in the property removes the QC requirement to be able to `Show` the generated values which makes it work.