Explicit object representation testing

Our test suite currently tests for most (but not all) core cf objects that the inspection (__repr__, __str__ and in cases where applicable, dump) methods don't error, without going further and checking that they give the desired output explicitly, typically something close to this:

https://github.com/NCAS-CMS/cf-python/blob/63ffeae84c2d9e479d52b499c60b910cc99e75c6/cf/test/test_AuxiliaryCoordinate.py#L56-L60

It would be nice, ideally, to test that the outputs are correct and exactly as they should be, to capture representation issues and typos such as that raised in #279 (which I fixed without adding a dedicated test to capture it because most, if not all, of our representation tests lack such an explicit test and it would be best to get such tests all added as a batch, I think).