Add data validation tests to CI

[jklmnn]

After merging #171 I noticed that Bonn was marked invalid on https://jklmnn.github.io/parkendd-status/. I think we should add data validation tests to the CI chain.

The invalid data was caused by the default of free lots to be "unbekannt" instead of an integer as defined by https://github.com/offenesdresden/ParkAPI/wiki/city.json. Although I have to admit this document seems to be outdated. The current checks of the check page are done against https://github.com/jklmnn/parkendd-status/blob/gh-pages/schema_city.json.
Nevertheless the invalid data cannot be handled by the Android App so I fixed this behaviour in c951c72.

@Mechtecs I assume you either used this default due to missing documentation or because you wanted to distinguish between full and unknown lots? In the first case you could pull this fix into your fork. Generally such a distinction would be helpful yet incompatible to the current API.