Adjust tests that fail due to vectorized accumulation

Commit c7f3054 changed the order of summation in inner products and norms because
these operations are now done in vectorized form. The affected tests showed spurious
changes in the last digit.

To improve, the following changes were made:
- error_estimator_0?: The output was scaled by l1 norm which is unsafe. These tests
  should only test error estimation and hence the scaling is done by linfty_norm
  which will never depend on roundoff. Note that we output up to 7 digits of a float
  which notoriously runs into roundoff issues.
- mesh_3d_12 also outputs 6-7 digits of a float. Changed the accuray of output to 5
  digits.
- q_dg0_projection was deleted because it FE_Q_DG0 has a singular mass matrix and thus
  the final solution on the face can be anything if no further constraint is added which does
  not make sense within this framework. Note e.g. how different the previous
  solutions on the face for Intel/non-Intel compilers were...