Make Shelley test examples concrete

Currently the functions in [Shelley examples](https://github.com/input-output-hk/cardano-ledger/tree/master/eras/shelley/test-suite/test/Test/Cardano/Ledger/Shelley/Examples) are parametrized by era even though the only valid instantiation of these era parameters is `ShelleyEra C_Crypt`. We should make these signatures concrete so that we don't have to thread constraints through the test libraries like in #3045.

If there are functions that can be used across eras, then we should move those into generic tests in `cardano-ledger-test`.