Setup unit tests routine

[alkurbatov]

We have lots of code which could be tested in some way, e.g. the new filters. Unfortunately, the only tests we have are functional requiring launch of a game client.

This task is about setting up a number of small unit test suites to check at least the filters.

[alkurbatov]

Well, the simplest way possible is to add GoogleTest as submodule and drop all that hand-maderoutine the Blizzard devs wrote. While this will be not real unit-tests (we need a running client for run), we can add basic unit-tests, too.

[Nickrader]

https://github.com/Nickrader/cpp-sc2/tree/dev_unittest

I started off trying to implement a unit test routine. I have since moved the goalpost to completing a sample, covering the different test types in the GTest framework.
[ it's taking me a long time to translate between Google documentation and our API, since it's my first go at GTest and real-world code].

Think I'm far enough to open it up for feedback/future direction. Figured getting a sample done first would make it more accessible for future contributors.

Currently still a work-in-progress.

P.S. I've learned more about CMake recently, so I'll have to go over that again.

[alkurbatov]

First of all, this is definitely a good attempt and nice way to get useful experience.
Regarding the code the original idea is the following:

1. Link with googletest (seems that we should use FetchContent here as I saw quite a lot of problems with "simple install googletest").
2. Use the same flag (BUILD_API_TEST) to build all kinds of tests (requires large rewrite).
3. Replace custom test routine in the integration tests with google test things.
4. Add some unit tests (e.g. for unit filters) just to test that it works.
5. Add unit-tests run into github CI.

I am totally ok if you wish to implement some points from this list as whole list could take a while.

[alkurbatov]

By the way, perhaps you may find Catch2 more convenient to use in tests:
https://github.com/catchorg/Catch2/blob/devel/docs/tutorial.md