Enable SSD models from TensorFlow with OpenCL plugin of Intel's Inference Engine

modules/dnn/test/test_layers.cpp

@@ -877,6 +877,7 @@ TEST_P(Layer_Test_DWconv_Prelu, Accuracy)
     int shape[] = {1, num_input, 16, 16};
     Mat in_blob(4, &shape[0], CV_32FC1, Scalar(1));
 
+    net.setPreferableBackend(DNN_BACKEND_OPENCV);
     net.setInput(in_blob);
     Mat out = net.forward();