Merge branch 'fix/2.x-co2-tests-for-dune-2.6' into 'releases/2.12'

[test] fix co2 tests for Dumux 2.x and Dune 2.6

See merge request !963 (merged)

(cherry picked from commit a55f8413)

08da3c23 [test] fix co2 tests for Dumux 2.x and Dune 2.6