Break up checks into smaller methods

[fklein-lu]

Currently every check class contains a check() method, that executes all the different checks.

Having a large method that often looks for very different issues makes checks different to read and understand.

It also makes it impossible to not run certain checks on a more granular level.

We could use reflection to load the methods contained in a check class. We could then pass the array of obtained method names through a filter, to allow tests to be removed. Finally, we could run all the remaining methods.

This could probably be done in a backwards compatible way. We could investigate how to make sure that only check methods get run, and not things like helper functions.