sigencoding_tests: improve test coverage

Summary:
We weren't checking a huge number of hashtype byte variations for 64-byte
signatures, though some hashtype checks are done in script_tests.

This prepares for adding a Schnorr-only sigencoding checker.

Depends on D3331

Test Plan: make check

Reviewers: deadalnix, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3264