WIP: cmake tests
Add facility to test cmake modules.
We write a lot of find modules and add them to dune-common or any other dune module. Not always is the find modules used in the dune-module where it belongs to. Nevertheless, it would be good to test these modules also if not called in any CMakeLists file in the regular cmake. One way of doing this it to use a custom cmake test command that configures, builds and runs an independent cmake project containing the find module to test.