Add unit testing

[InvisibleUp]

It would be beneficial to set up a unit testing framework here. The codebase is very brittle, and having tests would dramatically help with that.

Steps to take:

1. Select a unit test framework
2. Install the unit test framework, somehow
3. Write some basic tests
4. Set up GitHub CI to run the tests automatically

Possible test frameworks:

• cmocka
• Criterion
• GoogleTest
• rolling our own with a healthy dose of <assert.h>