Remove ad-hoc framework for generating reference tests

[jtraglia]

This PR is an extension of:

• Remove ad-hoc framework for generating vectors #4976

I have made several updates to the pytest reference test generator workflow & I cannot commit to Leo's branch. I have cherry-picked his commits from his PR. Additionally, this PR does the following:

• Remove the "nightly mainnet tests" workflow since that's essentially a duplicate of the "nightly tests" action now. I have double checked and it still runs unit tests which do not produce any reference tests.
• No longer run the tests twice in the release action. And I've decided to forgo running the framework tests as these are unlikely to be an issue; they are run on the master branch so it will be quickly noticed there.
• Move the default location of reference tests from ../consensus-spec-tests to ./reftests which I believe is more intuitive, now that the consensus-spec-tests repo is deprecated.

Closes #4976.