[testapp] Update testapp for new data,len arg order

The testapp call to endpoint_recieve was never updated for the great
data,len convergence due to the .def files. Fixed and working again.

src/user/testapp/main.cpp

@@ -134,7 +134,7 @@ main(int argc, const char **argv)
 
     size_t len = 0;
     while (true) {
-        result = j6_endpoint_receive(endp, &tag, &len, nullptr);
+        result = j6_endpoint_receive(endp, &tag, nullptr, &len);
         if (result != j6_status_ok)
             return result;