Remove dependency on Threads

[mristin]

Currently, threads are only used for testing framework (and not testing cases themselves). IMO, it's better to shift responsibility for running tests in parallel to the testing environment (make test -j8) and decouple it from the testing code.

Following a previous issue, #34:

I would also prefer a simple solution that doesn't require Threads. I've written a small unit test framework for CAF. With minor modifications, it's probably a reasonable drop-in replacement.

Is there a reason not to use something more common/standard like https://github.com/catchorg/Catch2? Catch2 is header-only and can be included directly into the repository so it adds no extra installation steps.

[mavam]

Is there a reason not to use something more common/standard like https://github.com/catchorg/Catch2?

I have no strong preference. I'm fine with using Catch2. But since I wrote the other unit testing framework (and it's also header-only), it came to mind first.