[test] Make grid variables instationary
- The test fails on buildbot due to an assert (the grid vars are not instationary)
However simply removing the assert still makes the test pass.
This raises the question if this should be an error or if we can find a better solution. I think we could also always set the previous volvars in
init to the current volvars and just not use them for stationary runs (memory overhead).
@DennisGlaeser any ideas for a better solution?