Update testing dev docs to GitHub Actions

[kaitlinnewson]

Update docs from Travis to GitHub Actions. Closes #1247.

[kaitlinnewson]

Hi @withanage, could you give this update to the testing docs a review? I've only changed parts referencing Travis, and haven't updated anything about Cypress (but if you see other things that need to be updated, let me know and I can try to incorporate those too).

Here's the current outdated live version of the guide for reference.

[withanage]

hi @kaitlinnewson
thanks for that, i will review the docs.

[kaitlinnewson]

@withanage I've noticed that this "Download artifacts" link at the top of a PR when a test fails does not always appear consistently - do you know why that is?

[withanage]

@kaitlinnewson actually, github should generate them automatically, do you have an example, where you could not see the Download link ?

[kaitlinnewson]

@withanage Nevermind, I think I see the confusion - this link to artifacts at the top only appears on the .../checks page, but then does not appear if you click on a job (but can be accessed in the build log instead). For example:

• "Artifacts" link appears here: https://github.com/pkp/omp/pull/1710/checks
• "Artifacts" link does not appear here, once I click on the failed test: https://github.com/pkp/omp/actions/runs/11127681718/job/30920572138?pr=1710

[kaitlinnewson]

@withanage Any additional feedback on these docs updates?

[withanage]

@kaitlinnewson
Looks great! Please go ahead with merge