cover taproot-tweaked signing with integration tests

[conduition]

Currently, ZcashFoundation#584 does not have any test coverage for taproot-tweaked signing.

This PR adds integration test coverage for signing with a commitment to a tapscript merkle root, proving that we can sign as a given group verifying key both with and without a tweak.

This chunk of code added to ciphersuite_generic.rs demonstrates how we can provide the 'effective' (tweaked) verifying key, AKA the 'taproot output key'. An observer can use it to verify signatures without the verifier knowing the tweak value used.

// Check that the effective verifying key can be verified against the raw message,
// without exposing the SigningParameters.
pubkey_package
    .verifying_key
    .effective_key(signing_target.sig_params())
    .verify(signing_target.message(), &group_signature)?;

I fixed a couple of minor mistakes in the first three commits. This commit specifically fixes the issue which @MatthewLM reported here.

[zebra-lucky]

git fetch origin pull/4/head:pr4
git checkout pr4
cargo test

[zebra-lucky]

Tests seems to be passed.
Looking to code changes.

[zebra-lucky]

I'm sorry, I'm to ill for now to make review of code changes.
But I don't want to be a stopper on these changes.
But if some bugs is remain, they must be found