Add UnitTest import option with category

[fuzzysolutions]

I added the ability to use the categories also for unit tests (and not only for manual tests), so that in the test selector, unit tests don't all end up under the category "general".

To make use of this option, the user needs to put a function unitTestsWithCategories() to his run_all_tests module, returning a list with tuples of tests and categories instead of a list of tests like in unitTests().

By the design of the code, the option is backwards compatible, if there is no unitTestsWithCategories(), the code still looks out for the unitTests() function.