Add test using Godot Engine loading the extension without error and doing something with it.

[Klaim]

The Godot Engine executable being small might be ok to put directly in git, or maybe just downloading it directly as part of the test is best. Just make sure it's the same version as the version of the godot-cpp package.

Then add as testscript to the gdexample the cli run of the godot engine after the generated binaries from the gdextension test are moved into the demo project (provided by upstream). There is a cli api for the godot engine that should be useful. There must be no error on startup of the Godot Engine, and then you should be able to generate and run the demo. Not sure how to handle how to test the generated demo, need to check.

[Klaim]

• Since last year godot-cpp as an actual test project in it's tests: https://github.com/godotengine/godot-cpp/tree/master/test
• There is a cmake template for godot-cpp based projects, maybe it would be useful to use it as a test too? https://github.com/godotengine/godot-cpp-template