Including a test project

Currently, with this repo alone, there is no way to test wether or not changes work, create leaks or even compile. The latter is partly because some features are header-only and only get compiled when a project uses them, so when we do PRs we have to test them on our projects.

I'm considering the possibility of adding a folder to the repo, in which we'd put a Godot project with a GDNative library inside, which would run a series of simple tests. I'm not necessarily thinking of a full unit test suite right away. At least having a simple project for the CI to compile and run would be a good start, so new PRs can automatically get better test coverage on multiple platforms.

The goals of this:
- Check if the header-only parts of the project compile
- Check if it runs without crashing
- Check if it doesn't leak memory

My only concern is how much of a deal is it to have these extra files. Perhaps it's not too bad considering the size it would take (about a hundred kilobytes?), compared to build symbols which are measured in hundreds of megabytes.