Fix AddTest
Created by: chleh
In #1239 I messed up the vtkdiff tests: One filename and two fieldnames are given, e.g.,
tes_zeolite_discharge_small_pcs_0_ts_19_t_0.100000.vtu pressure pressure
.
If the two field names are equal, that field is compared to itself yielding zero error norms.
This PR fixes that.
TODO: Parallel PETSc tests.