[QGIS-Developer] Changes to CI test setup

[Nyall Dawson]

Hi all!

Quick heads-up about some changes to our CI setup. Previously we
relied heavily on the cdash server run at
https://cdash.orfeo-toolbox.org/ to make images rendered during tests
available and visible for comparison purposes.

This hasn't been working well (well, at all) lately.

So I've just implemented some changes to the Github workflows so that
now any rendered images which cause tests to fail are now available as
an artifact after the tests complete. You can access these via the
"Checks" -> "QGIS Tests" page on the pull request, as the
"test-results" artifact.

Ideally we'd make these results a bit more visible (E.g. auto-creating
a comment on the PR with a link to the report, or even better
embedding the rendered image results directly as a comment to the
PR!), so if anyone has any experience in github workflows and wants to
give this a shot it'd be much appreciated by all!

Nyall