Remove common differ cases from subclass tests

The looping through test cases is bad (this will be explained more fully in the next commit). Removing them from the subclasses. They will later be reimplemented.
Looped through tests are bad

Instead of failing, they just don't run if the circumstances aren't right. For example here, a test wasn't running due to a very small typo: 0f29d677eca, or here because the code they are meant to be testing is no longer present: ebd330587. In both of these examples, the tests just aren't run. This makes them fairly useless as tests; when I remove code that is tested I expect the tests to fail.

This commit reimplements the tests as actual tests. They only test the configuration differ file.
Rewrite config differ subclass tests as tests

Rather than as loops, as per 733e00c.