IMPES with AMG test: fix the solution that is compared with the reference,...

IMPES with AMG test: fix the solution that is compared with the reference, consistent with the non-AMG test. Was not detected before, since it was only tested in parallel.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11809 2fb0f335-1f38-0410-981e-8018bf24f1b0