Avoid CMake looking at test dir if testing is disabled

This saves building the two test support programs.

CMakeLists.txt

@@ -160,7 +160,9 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
 
 add_subdirectory(src)
-add_subdirectory(test)
+if(BUILD_TESTING) # This option is defined implicitly by `include(CTest)`.
+  add_subdirectory(test)
+endif()
 
 set(man1 "man/rgbasm.1"
          "man/rgbfix.1"