Configure Tests/CMakeTests only with BUILD_TESTING ON

Since commit e03f83f3 (ProcessorCount test: fix path to cmsysTestsCxx
executable, 2013-01-24) the directory references a target that is not
built when BUILD_TESTING is OFF.  It makes no sense to add tests without
BUILD_TESTING anyway.

While at it, use add_subdirectory instead of subdirs to add CMakeTests.

Tests/CMakeLists.txt

@@ -2442,6 +2442,6 @@ ${CMake_BINARY_DIR}/bin/cmake -DVERSION=master -P ${CMake_SOURCE_DIR}/Utilities/
     set_tests_properties(KWStyle PROPERTIES
       WORKING_DIRECTORY ${CMake_BINARY_DIR}/Utilities/KWStyle)
   endif()
-endif()
 
-subdirs(CMakeTests)
+  add_subdirectory(CMakeTests)
+endif()