InvertCubicPolynomial test fails

In our pipeline, test_math regularly fails. According to GitLab it failed 4 times in the last 14 days - although I think we have the CI only since about a week. The invertCubicPolynomial test seems to be the cause of error, here is the output from tonight:

terminate called after throwing an instance of 'Dune::Exception'
  what():  Dune::Exception [operator():/builds/dumux-repositories/dumux/test/common/math/test_math.cc:282]: [invertCubicPolynomial] Root 0 of 3: -7.81946 does not match reference

The night before, for instance, it seems that it did not fail. @timok, I believe you implemented this? I haven't looked into the test yet, but could it be that the deviation threshold in the test is chosen too small?