Test cases should avoid dynamic non-test-relevant behavior

**Test cases should avoid dynamic non-test-relevant behavior**
We should change dynamic expressions like LocalDate.now() in tests to static constants in order to improve the reliability of the tests. Dynamic behavior should only be part of a test, if the test is especially built for such a parametrization.

_Originally posted by @gstuer in https://github.com/kit-sdq/TimeSheetGenerator/pull/79_