CMake: run X-dependent tests using a dummy Xorg server on make check-headless
This is possible using dummy-xorg-test-runner.sh to run these tests. Calling ./dummy-xorg-test-runner.sh binary --any-parameter -f --bar will load binary (with requested parameters) in a fake X server (using dummy xorg video driver, that now also uses llvmpipe to get 3d acceleration). Fixes: https://bugs.launchpad.net/bugs/1197561, https://bugs.launchpad.net/bugs/1199857, https://bugs.launchpad.net/bugs/1199890.