Fix virtual inheritance for class PadLayerTest (#3914)

inference-engine/tests/functional/plugin/shared/include/single_layer_tests/pad.hpp

@@ -28,7 +28,7 @@ typedef std::tuple<
 namespace LayerTestsDefinitions {
 
 class PadLayerTest : public testing::WithParamInterface<padLayerTestParamsSet>,
-                     public LayerTestsUtils::LayerTestsCommon {
+                     virtual public LayerTestsUtils::LayerTestsCommon {
 public:
     static std::string getTestCaseName(testing::TestParamInfo<padLayerTestParamsSet> obj);
 
@@ -36,4 +36,4 @@ protected:
     void SetUp() override;
 };
 
-}  // namespace LayerTestsDefinitions
+}  // namespace LayerTestsDefinitions