message: nydus-image/trace: skip registering trace class if it is registered
  Sometimes, UT tests are running concurrently registering the same trace
  class. So it is overrided. We should check if it already inserted.
diff: @@ -107,8 +107,12 @@ pub struct BuildRootTracer {
  impl BuildRootTracer {
  pub fn register(&self, class: TraceClass, tracer: Arc<dyn TracerClass>) {
  let mut guard = self.tracers.write().unwrap();
  + // In case a certain class is registered multiple times, e.g. from several
  + // concurrently running test cases.
  + if guard.get(&class).is_none() {
  guard.insert(class, tracer);
  }
  + }
  pub fn tracer(&self, class: TraceClass) -> Arc<dyn TracerClass> {
  let g = self.tracers.read().unwrap();
  