[FEATURE REQUEST] Add basic testing infrastructure and documentation

[AaronErhardt]

There should be some testing infrastructure for gtk4-rs apps and internal code that's reusable and documented.

At least something as a convenient entry point for running tests that rely on GTK (something similar to https://github.com/gtk-rs/gtk4-rs/blob/master/gtk4/src/lib.rs#L62).

I've seen gtk-rs/gtk-test#47 and to be honest, I don't have a lot of resources currently, too, but I hope we can at least work together to enable some basic functionality for the upcoming release.

[bilelmoussaoui]

cc @jf2048 as you wanted the test_synced to be public as well

[bilelmoussaoui]

I am not sure what would be needed other than making that function available + some examples. I wonder if the gtk_test_init would be any useful for this case?

[jf2048]

Yes, it seems like that needs to be public (or it needs to be copied) if someone wants unit tests for any gtk widgets, or for gtk4-macros, libadwaita-rs, libshumate-rs, libpanel-rs, sourceview5-rs, etc. Seeing a gtk4-test would also be good and probably not too hard.

Perhaps it should be made into a macro that calls test_synced internally? That would avoid making it part of the public API:

#[gtk::test]
fn mytest() {
    // ...
}

And would also allow supporting async tests similar to #[tokio::test].

[jf2048]

This issue is still a problem though: #811

Although maybe it's not that bad in practice since the tests all run synchronously anyway?