unit tests: make sure the results are correct

where correct means "identical to the reference solution". so far,
only a reference solution for the test of the 2p box model is provided

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7076 2fb0f335-1f38-0410-981e-8018bf24f1b0