[test][3pwateroil][sagd] do not use AMGBackend

On minimal setups where UMFPack/SuperLU are not installed, this test
fails as AMG then uses an iterative solver (as fallback) with a too soft
convergence criterion.
4 jobs for !2601 with fix/3p-tests-on-minimal-setup
latest detached
Status Name Job ID Coverage
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
passed minimal-dune-2.7-gcc

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