[WIP] Introduce a GitHub Actions workflow for running the integration tests

[timvandermeij]

This is a rough attempt to get the integration tests running on GitHub Actions. It's intended to get more experience with what kind of issues we run into before we can enable this. Proper fixes will be introduced in separate PRs.

Related to #11851.
Replaces #20029 (which accidentally got closed due to a branch deletion).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.64%. Comparing base (6541903) to head (dcedace).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20371      +/-   ##
==========================================
+ Coverage   62.63%   62.64%   +0.01%     
==========================================
  Files         169      169              
  Lines      120729   120729              
==========================================
+ Hits        75616    75631      +15     
+ Misses      45113    45098      -15     

Flag	Coverage Δ
fonttest	7.66% <ø> (?)
unittestcli	62.61% <ø> (-0.02%)	⬇️

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[marco-c]

@timvandermeij I see very few tests failing, could we disable them for CI so we can land this? And then address the failures in a follow-up PR?

[timvandermeij]

We're very close indeed. The tests here are mostly blocked by #20814. I have extracted #20828 to get that part in place first, and after that I think there's only some caching work to do here (which I hope to finish this weekend) and then we should be good to get this in place. Note that the changes I'm extracting in e.g. the mentioned PR also make porting the unit tests to GitHub Actions easier for a next step.