Rename variables and types 'trial' -> 'test'

In the classical terminalogy of Galerkin methods, the 'trial' space is the same as the 'ansatz' space. Searching the internet, this seems to be consistent and you cannot find cases where 'trial'-space is used for the test space.

Unfornunately, we used the term trial-space/basis/... where test was meant throughout dune-fufem. Once you notice this, it's quite distracting. This patch changes the naming throughout dune-fufem. Fortunatenly this only shows up in variable and template names and hence does not change any public interface.

One could also thing about next renaming 'ansatz' to 'trial' but IMO this is just a matter of taste, while the current change fixes an error.