test_mpfa2p.cc broken with Clang 12

Using Clang 12 I get an error, not sure why it complains…

In file included from /home/gruenich/dune/complete/dumux/test/porousmediumflow/sequential/2p/test_mpfa2p.cc:32:
In file included from /home/gruenich/dune/complete/dumux/test/porousmediumflow/sequential/2p/test_mpfa2pproblem.hh:50:
/home/gruenich/dune/complete/dumux/test/porousmediumflow/sequential/2p/test_mpfa2pspatialparams.hh:83:91: error: use of class template 'LinearMaterialDefault' requires template arguments
        return std::is_same_v<PcKrSwCurve, LinearMaterial> || std::is_same_v<PcKrSwCurve, LinearMaterialDefault>;
                                                                                          ^
/home/gruenich/dune/complete/dumux/test/porousmediumflow/sequential/2p/test_mpfa2pspatialparams.hh:51:7: note: template is declared here
class LinearMaterialDefault;