Improve test coverage

[MartinBenediktBusch]

Test Coverage Status Quo:

To Do

• [x] Make tests independent of the default seed as described here https://github.com/lexicongovernance/pluraltools-backend/issues/335
• [ ] Create test cases for services that do not have any test yet
• [ ] Push test coverage of functions (% Func) to 100% (i.e. each function should at least have one test case).
• [ ] Push overall percentage (row: all files) of Statements (% Stmts) and Lines (% Lines) to >= 80 %