Move away from TestEZ for unit testing

[dphfox]

While TestEZ has been a good help so far in ensuring the correct functioning of Fusion and giving more confidence about not accidentally introducing regressions, it's starting to get awkward to use it. Other than being an external dependency on Fusion which is something we try to avoid, it also relies on function environment manipulation and documentation is not easy to come by.

In general, it'd be nice to build a solid unified tool for testing Fusion with - we already have our own benchmarking solution which could use some polishing up, so we could explore building up our own framework for running the simple kinds of testing and benchmarking we use.

[dphfox]

A replacement for TestEZ is lined up to go! I'll work on converting everything over after v0.2.