[TASK] Create directory structure for making fine-grained tests (#754)

We'll rework and sort the tests into these folders:

- `Unit` and `Functional`: for tests that do not call deprecated methods,
  and where PHPUnit should warn if deprecated methods are called
- `UnitDeprecated` and `FunctionalDeprecated`: for tests that call
  deprecated methods, and where PHPUnit should not warn about those

For the time being, we still call all tests with the same configuration.

Author: oliverklee