MXS-1702: Clean up and convert test to C++

Cleaned up and updated the test; the code was written with a pre-C99
standard in mind.

Added a get() method into mxs::Buffer to make it easier to use with
non-C++ functions.

include/maxscale/buffer.hh

@@ -533,6 +533,19 @@ public:
         return *this;
     }
 
+    /**
+     * Get the underlying GWBUF.
+     *
+     * @return  The underlying @c GWBUF.
+     *
+     * @attention This does not release ownership of the buffer. The returned pointer must never be
+     *            freed by the caller.
+     */
+    GWBUF* get()
+    {
+        return m_pBuffer;
+    }
+
     /**
      * Resets the underlying GWBUF.
      *