[test] test_1pni_conduction_tpfa, test_2p_incompressible_tpfa_oilwet and test_3pni_conduction_tpfa fail with minimal setup

test_1pni_conduction_tpfa and test_2p_incompressible_tpfa_oilwet and test_3pni_conduction_tpfa fail with dune master with a strange (maybe solver-related) problem when dumux is run in minimal configuration (no MPI, no UMFPACK, nothing external, only dune core modules).

This may be related to the coarse grid solver of the AMG, which is usually UMFPACK but changed to SSOR when UMFPACK is not found. The SSOR has a low hard-coded solver tolerance. This solver tolerance is adjusted with a patch in dumux/patches/dune-istl-2.6.patch. The patch is supposed to address this issue and worked in the past. But doesn't seem to work now....

versions

• Dune 2.7 or master
• Dumux 3.2 or master