Add automated tests that run a GDExtension (rather than just building it)

[dsnopek]

In order to prevent regressing or reverting old fixes, this PR adds some automated tests to the CI that involve actually running a GDExtension.

Some highlights:

• It converts the project in 'test/' to be just an integration test NOT also an example for users. If we want to have a true example project, personally, I think it should be in its own repo, and it should be pared down to just showing the things we really want beginners to know about. At the moment, it has a little bit of that, but it's also just doing things to make sure they work. Let's split them! (This will also require changing the 2nd paragraph on this docs page, that recommends this as an example: https://docs.godotengine.org/en/latest/tutorials/scripting/gdextension/gdextension_cpp_example.html)
• It adds a simple test harness for asserting things. This isn't very complete but can be improved over time. I don't want to get carried away with this in the first PR.
• It turns everything that can be made assert-able in the original demo project, into assertions. A handful of little things couldn't be converted (so for those we're really just making sure we're not crashing :-)), but I think anything is better than nothing for now. Like the previous point, we can improve this over time.
• It adds some tests for the bug fixed by Fix crash using Ref<T> as parameter #1045. I had actually intended to add a test for the regression caused by that PR, but I didn't consider that that would be a failing test. :-) So, while we work on fixing that regression, the tests here will make sure we don't bring back the bugs in Crash when passing Image to a function  #1021 and Crash using Ref as parameter #1024. Here is a patch adding a test that does show the regression, which we can add after it's fixed.

This is a DRAFT because there's a couple I'd still like to figure out:

• It's downloading Godot v4.1-dev1 to run the tests - it'd be better if it grabbed the latest artifacts from 'master'. There's a GitHub action that should allow us to do this, but I haven't gotten it working yet. See It's trying to download a very old artifact, when much newer ones exist dawidd6/action-download-artifact#240
• I want to add tests for the issue from PR Fix crash using Ref<T> as parameter #1045. This would make these new tests immediately useful for a problem that we're actively trying to tackle right now.

Please let me know what you think!

[dsnopek]

I got it downloading Godot from the latest artifacts on the 'master' branch!!

I had to patch action-download-artifact: dawidd6/action-download-artifact#241

[dsnopek]

Alright, I added some tests related to #1045. They actually test the issue that #1045 fixed, since those tests will pass - it doesn't test the regression caused by #1045 because that would fail

Attached to this comment is a patch adding a test that does show the regression, but I guess that'll have to merged with a fix, so that it passes :-)

At this point, there's nothing left that I want to add here - taking out of "draft" status!

test-pr1045-regression.patch

[saki7]

As for the Ref<T> issue the current approach looks solid. I completely agree that we can gradually improve the tests; this PR would be a great first step.

[dsnopek]

Discussed at the GDExtension meeting: looks good, except rename the "demo" folder to "project"

As far as an actual demo: we dicussed cleaning up https://github.com/paddy-exe/GDExtensionSummator and promoting it to be an official example in the godotengine organization

[dsnopek]

My latest push renames the "demo" directory to "project" as requested at the meeting!

[dsnopek]

The last test on this PR didn't pull the most recent artifact from Godot, but one from the day before, which has me a little worried:

(The test ran on 2023-05-14 but the artifact is from 2023-05-13.)

I'm going to do some additional tests to make sure there isn't some issue with the GitHub Action. We really need this to grab the latest artifact for situations where we need to coordinate merges between Godot and godot-cpp.

[dsnopek]

Alright, I added some code to the GitHub action that should ensure that we're always getting the latest artifact:

dsnopek/action-download-artifact@1322f74

It seems to be working in my testing, and also in the latest CI build on this PR!

So, I think this PR is (once again) ready :-)

[BastiaanOlij]

Discussed in GDExtension meeting, we're all happy with these changes, should be merged.

[dsnopek]

Blergh. This just had a run on PR #1113 where it pulled an old expired artifact. :-/ I ran this a whole bunch of times on my fork and it was working consistently there! I may need to dig back into this one...

Here's the output from that run: https://github.com/godotengine/godot-cpp/actions/runs/5080843000/jobs/9128373154