[GTK4] Get tests running on GitHub actions

[jonahgraham]

We've had GitHub actions for GTK4 for a while now, but we've never had a complete test run. There are numerous reasons I have already identified:

• callback leaks (such as [GTK4] Add missing Callback dispose in Display #2703)
• memory leaks (such as [GTK] Fix memory leak, GUri was not freed. #2704)
• known failing tests obscure regressions
• missing dependencies (such as no webkit for GTK4)
• permission problems with new isolation around apparmour + webkit
• run tests on wayland (currently uses x11)

This is the umbrella bug for me to attach all the PRs I plan to do in this area to. I have started work on each of the areas in the above list in #2690 while trying to fix #2667 but decided that I needed to split this out to a separate task now. PRs incoming over the coming days on this topic.

[jonahgraham]

My goal for this issue is that GTK4 tests are on for all PRs alongside GTK3 and other tests so that we can ensure other changes that are not explicitly GTK4 don't regress progress in GTK4.

[akurtakov]

I have #2600 as a test for this. Feel free to use/rebase it for testing purposes.