scripts: fix warning in test util

CI reports:
In scripts/test-util.sh line 216:
    cleanup() {
    ^-- SC2329 (info): This function is never invoked. Check usage (or ignored if invoked indirectly).

The shellcheck can't trace calling in trap, so we need add hint
to make it happy.

Signed-off-by: Yi Wang <foxywang@tencent.com>

scripts/test-util.sh

@@ -212,7 +212,7 @@ mount_and_exec() {
     local COMMAND_STATUS=0
 
     # Cleanup function to unmount and detach loop device
-    # shellcheck disable=SC2317
+    # shellcheck disable=SC2317,SC2329
     cleanup() {
         if [ -n "$MOUNT_DIR" ]; then
             echo "Cleanup: Unmounting $MOUNT_DIR..." >&2