test: Streamlined the config-IT, so that they use the same expectations

test: Streamlined the config-IT, so that they use the same expectations

- Both tests now use the same data and expect the same results.
- Moved the actual tests and assertions to `AbstractConfigurationIT`.
- Also streamlined `AbstractStorageStrategyIT`, so that it produces
  the exact same data, that the configuration-tests are expecting.
  This is not a necessary change to get the refined coniguration-tests
  running. It just makes it easier, to produce the needed test-data for
  future refinements.
- Also streamlined the naming of the directory, where
  `InMemoryWithFilesStorageIT` stores its data.