Silence warning "value stored to rv is never read"

tests/utils.cpp

@@ -8,6 +8,7 @@ void prepare_logdir()
     system("del /F /Q logs\\*");
 #else
     auto rv = system("mkdir -p logs");
+    (void)rv;
     rv = system("rm -f logs/*");
     (void)rv;
 #endif