Add support for unit tests

[hhyyrylainen]

• Godot requiring tests (opened separate issue: Add Godot-requiring unit test support #4963)
• plain c# requiring tests

Until godot adds proper support we should maybe use nunit3 directly.
Godot proposal: godotengine/godot-proposals#432 (go thumbs up this)

[rhumborl]

I have dropped nunit3 in to the solution and done some initial investigation.
See https://github.com/rhumborl/Thrive/blob/1571_cs_unit_test_support/Thrive.Test/DummyTest.cs

1. Unit tests must be stored separate from the main project (probably a good idea anyway).
   • If not, tests are not discovered by the console runner.
   • I was running it in Visual Studio 2019 - Test Explorer discovered them but Run All Tests came up with 0 tests found.
2. Tests can reference code in the main Thrive assembly and run successfully for non-Godot code.
   • This should mean the majority of the core engine can be made testable.
3. If any Godot code is executed during a test it will fail, as it is run outside the Godot environment.
   • This produces a similar result to the OP in Add or document a way to write unit tests for Godot C# projects (e.g. with nunit3) godotengine/godot-proposals#432

Best suggestion I have currently is to make UI components as much a facade as possible, i.e. abstract Godot classes such as InputEvent to our own versions and pass them to core classes instead.

If a universally-executable runner doesn't appear as van800 was looking at in the proposal, you'll need to discuss how much test coverage vs. refactoring you might need to do.