CMake build system: Fix the "testclean" target Regression caused by f9134384b728d8943f252b27464d83c4b7b2d159 This commit also makes the "testclean" target clean up the 4:1:1 test images. This was implemented in the autotools build system in 1f3635c4969f2319a01c9fe561958815b733227f but was left out of the CMake build system due to an oversight.