Cleanup/rename 2p xc tests

please be advised that the 2p2c changes triggered some changes to the mpnc-comparisons and 2p-ni files.

Also: The 2p2c waterair and injection test are separated now, as it is according to the guideline. The waterair test is a non-isothermal test. So far the naming does not indicate that. If we want this to be clear we could change it to "waterairni" or "niwaterair" or simply "nonisothermal". I did not like any of those, but maybe someone has a strong opinion.