Separate tests into unit / integration / ... directories

[hakre]

To improve the speed how tests can be run - especially the unit-tests - I suggest to divide tests into sub-directories, for example:

unit - for unit tests
integration - for integration tests
functional - for functional tests

Right now all tests are roughly labeled as "Unit-Tests" even they are not unit-tests per-se, e.g. some of them interacting with the database or the file-system.

Background: ​What's the difference between unit, functional, acceptance, and integration tests?

[bpetty]

Discussed this ​in IRC, and decided this isn't a realistic goal as most tests can't actually be clearly categorized like this right now.