Refactor compatibility issues

[tobixen]

Move it from the tests directory to the caldav directory.

Rationale:

• Work on the server checker script. It should not have to import things from the test directory - and now I'm also splitting it out as a separate repository/project, then it's not possible even to have it read from the test directory
• When I start working at Server compatibility hints #402 I will also need to have this data available as part of the actual library