[cmake] Find runtest script from dumux_INCLUDE_DIRS
dumux_INCLUDE_DIRS is a cmake list of paths and the runtest script should be searched for within those paths. Prior to dune 2.10 the variable in practice contained only one path.
dumux_INCLUDE_DIRS is a cmake list of paths and the runtest script should be searched for within those paths. Prior to dune 2.10 the variable in practice contained only one path.