Migrate keygen and signing tests to be non-generic (ciphersuite-specific)

[conradoplg]

We have generic tests in frost-core which are called by the ciphersuite crates, parametrized with their ciphersuite types.

However that does not test if the ciphersuites re-expose all the required types correctly.

We can instead have ciphersuite-specific tests which are replicated to all ciphersuites with gencode. It should be easy by simply taking the generic tests and "de-parametrizing" them. See #401 for an example.

As an upside that will greatly increase our test coverage too and will end up closing #233.