Remove & Retention: Replace "fake" unit tests with real ones along refactoring the production code to make it testable

[buhtz]

Describe the problem, feature or ask a question:

Currently the existing "unit tests" related to Remove & Retention are "fake" because they do not test the productive code but code surrogates.

The task are, to refactoring the productive code to make it testable. Adapt the existing unit tests to the productive code.

Meta issue:

• #1945