Looped through tests are bad

Instead of failing, they just don't run if the circumstances aren't right. For example:
 - 0f29d677eca: one of tests wasn't running due to a very small typo
 - https://github.com/alphagov/vcloud-core/commit/ebd330587: the code they were testing is no longer present, so the tests just don't run; when I remove code that is tested, I expect the tests to fail.

This makes them fairly useless as tests. This commit reimplements the tests as actual tests. They only test the configuration differ file as this stage.
Rewrite config differ subclass tests as tests

Rather than as loops, as per 9883ea53ef2.