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/modules/filter/test/maxscale/mock/session.hh

@@ -13,10 +13,11 @@
  */
 
 #include "mock.hh"
-#include <maxscale/session.h>
 #include <maxscale/protocol/mysql.h>
 #include "client.hh"
 
+#include "../../../core/internal/session.hh"
+
 namespace maxscale
 {
 
@@ -27,7 +28,7 @@ namespace mock
  * The class Session provides a mock MXS_SESSION that can be used when
  * testing.
  */
-class Session : public MXS_SESSION
+class Session : public mxs::Session
 {
     Session(const Session&);
     Session& operator = (Session&);