Add CI for regression tests

[stellaprins]

What does this PR do?
Resolves #23 by running the regression tests in the GitHub Actions workflow.

Generalises runUnitTests functions for reuse with regression tests.

• runUnitTests_show_results ->show_test_results
• runUnitTests_create_artifact ->save_artifact

Runs regression and unit tests in the same workflow to create a clear overview

• unit_tests.yml -> run_tests.yml

See the new CI output here.

Linked issues
#9 #23

Notes
Adding both the unit and regression tests to the same run_tests.yml makes the yml file a little bit more unwieldy but results in a much nicer CI output compared to seperating them. I tried to avoid some of the repetition in the yml file with the use of anchors but decided the code was clearer as it was without anchors.

[bencox]

Looks very useful, thank you.

[stellaprins]

Thanks @bencox! I made one last tweak. I realised the CI output opens at the bottom when it fails, so it actually makes more sense to put the most important info (N failed / passed & failed tests) at the end. I’ll merge once I’ve confirmed the CI output looks right.