Make the entire test suite pass

As an important step towards using Gitlab CI for our project, we should dig into those test cases that currently fail for several reasons. The tests currently are:

• All combinations of instationary and explicit: The problem seems to be with PDELab
• poisson_dg with matrix free: It is very likely we do not generate code for a suitable solver
• poisson_dg with neumann boundary conditions: The reference solution is from the all-Dirichlet case. In the CG case it is okay to reuse it, that might differ for DG.
• stokes_symdiff, stokes_dg_symdiff and stokes_stress_numdiff: Thresholding problem in dune-testtools
• stokes_stress_symdiff: Pressure is multiplied with -1