face+util: Add protected Face::getTransport method and simplify DummyClientFace implementation

Change-Id: I42e7e5705f27b6823ed18107b374c9841d2a7500
Refs: #3146
diff --git a/src/face.cpp b/src/face.cpp
index 0212fd7..a4f3615 100644
--- a/src/face.cpp
+++ b/src/face.cpp
@@ -133,6 +133,12 @@
 
 Face::~Face() = default;
 
+shared_ptr<Transport>
+Face::getTransport()
+{
+  return m_transport;
+}
+
 const PendingInterestId*
 Face::expressInterest(const Interest& interest,
                       const DataCallback& afterSatisfied,