[cleanup] Use new ignore field feature of the test driver

The dumux test script https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/bin/testing/runtest.py gets with !3353 (merged) an option to ignore fields (--ignore "fielda" "fieldb"). This can be used instead of setting a very high zero threshold to make the intent clear.

Example: rank should be ignored when comparison parallel and sequential files. Example implementation: test/freeflow/shallowwater/dambreak/CMakeLists.txt