bsim: Bluetooth: mesh: fix model extension

In 'test_access.c', model 3 on element 1 extending model 3 on element 0
is now registered correctly. When calling 'bt_mesh_model_extend' on
models on different elements, this needs to called from the
model with the highest element index to be registered correctly.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>