[BugFix] shear wave test

The lattice Boltzmann step tries to figure out a stencil on its own, when no stencil is provided in the config class. This did not work in the shear wave test. The suggestion here is to just state the stencil in the config file

added Bug label

requested review from @schottenhamml

assigned to @holzer

approved this merge request

merged

mentioned in commit b9a09386