Add subcontainers to registry for serialize unit test
Add subcontainers to registry for serialize unit test

The serialize function doesn't save the subcontainers recursively. It merely saves IDs. It finds the IDs from the registry when deserialising, so it must have the containers in the registry first.
This also prevents the need for deep-checking those subcontainers. Just check for array equality.

Contributes to issue CURA-1278.

onitake/Uranium,onitake/Uranium