Convert to Catch2 testing framework

[droidmonkey]

Summary

Catch2 is a header-only test framework that is very slick. Converting to Catch2 would allow us to eliminate the multitude of test executables while still clearly sectioning tests into discrete CPP files. Tutorial:

https://github.com/catchorg/Catch2/blob/master/docs/tutorial.md

Clion supports Catch2: https://www.jetbrains.com/help/clion/catch-tests-support.html

Examples

Using an QEventLoop with Catch2: https://stackoverflow.com/a/56569699

Sample Qt Project using Catch2: https://github.com/datho7561/qt-todo/tree/master/testing