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.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c7739c7..48a74bd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -866,7 +866,7 @@
 
 endforeach()
 
-add_custom_target(testclean COMMAND ${MD5CMP} -P
+add_custom_target(testclean COMMAND ${CMAKE_COMMAND} -P
   ${CMAKE_SOURCE_DIR}/cmakescripts/testclean.cmake)