Improve tests by asserting expected outcome

[mertd]

The current test only does a pretty basic check of whether the cleaning function exits cleanly. However, this would be the case even if e.g. not a single app was cleaned as long as the function itself does not throw an error.

Ideas:

• use an executable that should be in the path for every dev and/or runner environment and confirm it was executed
• add a mock app and confirm that it was called with the correct arguments