Add unit tests

[Flynsarmy]

This repo could really use a comprehensive suite of unit tests. Without them we're kinda flying blind.

[Flynsarmy]

EDIT: Ignore this comment. See #16 (comment)

---

I've been working on a unit testing PR but am having issues compiling. I'm not very good with C++ and have gotten stuck. Here's what I've got so far.

After pulling in the submodules, you can compile with scons target=template_debug --tests_enabled. The unit testing framework I went with is actually working wonderfully however when I try to test UtilityAI classes specifically, I get the following compilation error:

Compiling shared src\register_types.cpp ...
C:\path\to\Utility_AI\tests\test_bt_fixed_result.hpp(13): error C2065: 'UtilityAIBTFixedResult': undeclared identifier
C:\path\to\Utility_AI\tests\test_bt_fixed_result.hpp(13): error C7568: argument list missing after assumed function template 'Ref'
C:\path\to\Utility_AI\tests\test_bt_fixed_result.hpp(14): error C3861: 'print_verbose': identifier not found
scons: *** [src\register_types.windows.template_debug.x86_64.obj] Error 2
scons: building terminated because of errors.

Here's the exact line causing the issue https://github.com/Flynsarmy/Utility_AI/blob/b85508939e5d066ee6def349f4851acb0402554b/tests/test_bt_fixed_result.hpp#L13

You can see the unit testing framework in action by commenting out lines 13-14, compiling and running

godot.exe --headless --quit --path "C:\path\to\Utility_AI\demo"

If you've got a spare moment I'd really appreciate some help getting a single test up and running after which I can start working on more.

[JarkkoPar]

Thanks for raising this and for your great work with all the other contributions! I have quite a lot going on just now with work but I will try and arrange some time for this next week.

[Flynsarmy]

I'm now thinking a better and much simpler approach is to use gdunit4 and have our tests in the demo project. That way there's no fancy compilation steps or anything and it would make writing tests more accessible to non-C++ users.