[cmake] Fix broken input file links

It was checked in add_dumux_test whether the to be linked input file exists
but then another file was linked in some occations. 
Use the path relative to the source directory to fix the broken links.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15326 2fb0f335-1f38-0410-981e-8018bf24f1b0