test: simplify validators initialization

[unnawut]

🗒️ Description

Now that we have validator key caching, I think we can get rid of the 2-step validator setup. i.e. instead of 1) creating validators with mock pubkeys at the test case level, then 2) generate and replace the mock pubkeys with actual ones at the base fixture level, we can simplify the test case to provide just num_validators and let the base fixture handle the key management.

This helps simplify the test setup and drops one more test-specific behavior (no more zerofill mock pubkeys for all test vectors). Also a cleaner base for creating test vectors that verify signatures.

This does not change the behavior of any existing test so no new tests added.

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[tcoratger]

Thanks :)