Extend unit testing documentation

[Rayne]

A user had questions about unit testing but the documentation has no complete overview. I gathered some ideas and snippets:

• [ ] Copy/Move information from bcosca/fatfree#unit-testing to F3Community/F3com-data
• [ ] Extend https://fatfreeframework.com/3.6/test where necessary
• [ ] Explain how to run the F3's unit tests ~~(dev branch of bcosca/fatfree)~~
• [ ] It could also be useful to document how F3 can be combined with other testing tools, e.g. see my answer at Unit Testing with Fat-Free-Framework which utilizes PHPUnit.