Testing framework

[natinusala]

A testing framework for borealis should allow running "unit tests" of a single function's output, as well as "functional tests" that require creating an application, setting it up and running it for multiple frames to ensure the desired behavior on a higher scale.

The idea is to test both "units" (such as i18n, storage...) as well as app behaviors such as "when you click on a sidebar item and come back the focus should be on the current tab", which requires running the app for a set amount of frames and "hook" it.

Catch2 and trompeloeil looked promising as a base.