Make test more robust by
Decrease runtime by only using 1 cell in y-direction. We test in 2D to test the model in 2D but the specific setup is one-dimensional so the added cells can be spared.
The new reference solution was created with a timestep of 5s. The test uses a timestep of 25s which still reproduces and accurate enough solution so the comparison passes.
Fixes #1158 (closed)