Expand GitHub Actions test matrix

[nbougalis]

When running unit tests with PRs, the new runners appear to try and build only release mode code. That's not bad, but it misses out on opportunities to capture both compile time issues (e.g. compilation errors in debug-only code) and runtime issues (e.g. potential assert failures that are intentionally swallowed up in release builds, or debug-only code both in libraries and our own code).

Fix: unit tests should be executed in both debug and release configurations. I guess this is probably an issue for @legleux.

[thejohnfreeman]

Maybe we can use this issue to spec out the full matrix of configurations we want to test. I'll do some digging and come back with what I think matches what we had or were trying to do months ago.

[thejohnfreeman]

I found these resources:

• Build and test rippled in Github Actions, plus some related fixes and improvements #3851 : @ximinez's PR to add a matrix of GitHub Actions based on the old build system. The README gives a concise overview of the matrix.
• Add GitLab CI configuration #2897 : My PR to add a matrix of GitLab jobs, based on the old Jenkinsfile. This comment on that PR captures the output of a run of that Jenkinsfile, whose matrix is linked from the PR description. The .gitlab-ci.yml tries to copy that matrix.

I think the matrix in the README for #3851 is the one we want to implement. Before I start on that, I'm going to work on making sure our self-hosted runners are ready.

[thejohnfreeman]

As mentioned in #4454 (comment), we should add tests across the Docker images that we share (for now in https://github.com/thejohnfreeman/rippled-docker/).

[intelliot]

• next step is macOS test matrix
• no functional environment for Windows
• get test run times down
• opportunity for someone else to pick this up

[thejohnfreeman]

Add tests across compiler versions too.

[intelliot]

@thejohnfreeman at your convenience, could you comment here with an update on this issue? Any blockers?