test: Bluetooth: Mesh: additional model extension checks

This PR add more checks for model extension functionality for BLE Mesh.
It adds checking of:
* Models across multiple elements (should not share subscription lists)
* Sibling models (if models that extend the same model share
  a subscription list)
* Subscription list entry allocation (models within extension list
  use subscription list capacity of each other).

Signed-off-by: Aleksandr Khromykh <Aleksandr.Khromykh@nordicsemi.no>