MXS-1929: Fix mock testing framework

The testing framework extended the public struct, not the private
one. Also moved the internal Session class inside the mxs namespace to
prevent conflicts with the mock testing Session class.

server/core/internal/session.hh

@@ -48,8 +48,6 @@ struct RegistryTraits<MXS_SESSION>
     }
 };
 
-}
-
 typedef struct SESSION_VARIABLE
 {
     session_variable_handler_t handler;
@@ -122,4 +120,6 @@ private:
     DCBSet            m_dcb_set;         /*< Set of associated backend DCBs */
 };
 
+}
+
 std::unique_ptr<ResultSet> sessionGetList();