#1525 Avoid defining a "Model" for tests

So far, may tests are defining a mock model primarily just to initialize properly stuff.

However, for most of them we just need to:

• Initialize properly the singletons with the data from the input data file.
• Call initialization of GodOfDof, the class which does the bulk of the work.

So the model is really overkill in this case...

I have therefore tried to avoid using one for one specific case: the PetscVectorIO test, as a proof of concept.

It was quite a natural follow-up to #1524 (closed), and helps in a task in progress within #1443 (closed): clean-up the initialization API, which is a bit messy and was for the last years considered as a kind of black box.