message: dnn(test): test at least CPU code path
diff: @@ -276,6 +276,8 @@ static testing::internal::ParamGenerator<tuple<Backend, Target> > dnnBackendsAnd
  targets.push_back(make_tuple(DNN_BACKEND_OPENCV, DNN_TARGET_OPENCL_FP16));
  }
  #endif
  + if (targets.empty()) // validate at least CPU mode
  + targets.push_back(make_tuple(DNN_BACKEND_OPENCV, DNN_TARGET_CPU));
  return testing::ValuesIn(targets);
  }
  