Set up XUnit

[AdamNorberg]

It looks like we're okay with XUnit for testing, so it's up to me to set it up. I'll consider it "set up" when I can run a "hello world" test from MSBuild, since that's what the Godot directions prefer. After that, I'll set up a GitHub Action to automatically build and run all unit tests on every pull request or commit to master. Then, well, I've got some unit tests to write!

[AdamNorberg]

It turns out the setup required is negligible - simply declare the dependency and NuGet does the rest of the work, and my IDE sets up the dependency for me in the .csproj file. It just took me a while to learn that unit tests have to be in a separate "project" from the rest of the code.

That, however, poses a problem, because the current layout is not "multiple-projects"-flavored. #23 describes what needs to be done to make this work.