Move QPY tests to GitHub Actions and increase inter-symengine tests

[jakelishman]

Summary

This commit has two major goals:

• fix the caching of the QPY files for both the main and stable/* branches

• increase the number of compatibility tests between the different symengine versions that might be involved in the generation and loading of the QPY files.

Achieving both of these goals also means that it is sensible to move the job to GitHub Actions at the same time, since it will put more pressure on the Azure machine concurrency we use.

Caching

The previous QPY tests attempted to cache the generated files for each historical version of Qiskit, but this was unreliable. The cache never seemed to hit on backport branches, which was a huge slowdown in the critical path to getting releases out. The cache restore keys were also a bit lax, meaning that we might accidentally have invalidated files in the cache by changing what we wanted to test, but the restore keys wouldn't have changed.

The cache files would fail to restore as a side-effect of ed79d42 (gh-11526); QPY was moved to be on the tail end of the lint run, rather than in a test run. This meant that it was no longer run as part of the push event when updating main or one of the stable/* branches. In Azure (and GitHub Actions), the "cache" action accesses a scoped cache, not a universal one for the repository ^1. Approximately, base branches each have their own scope, and PR events open a new scope that is a child of the target branch, the default branch, and the source branch, if appropriate. A cache task can read from any of its parent scopes, but write events go to the most local scope. This means that we haven't been writing to long-standing caches for some time now. PRs would typically miss the cache on the first attempt, hit their cache for updates, then miss again once entering the merge queue.

The fix for this is to run the QPY job on branch-update events as well. The post-job cache action will then write out to a reachable cache for all following events.

Cross-symengine tests

We previously were just running a single test with differing versions of symengine between the loading and generation of the QPY files. This refactors the QPY run_tests.sh script to run a full pairwise matrix of compatibility tests, to increase the coverage.

Details and comments

This is a CI change, so the chance I got it right first time is approximately zero. Just need a PR to start testing it. We'll need to update the branch-protection rules if we decide to merge this.