TestDriver: use `CM_NULL` to avoid lints about `nullptr` usage

Templates/TestDriver.cxx.in

@@ -122,7 +122,7 @@ int main(int ac, char* av[])
   if (run_all == 1) {
     clock_t t;
     int status = 0;
-    const char* status_message = NULL;
+    const char* status_message = CM_NULL;
     printf("TAP version 13\n");
     printf("1..%d\n", NumTests);
     for (i = 0; i < NumTests; ++i) {