[Task] Increase unit test coverage of create cluster - brought subnet topology

[Skarlso]

Part of #5315

Right now, create cluster unit test works by generating the infrastructure including the subnets and the VPC.
A next step would be to be able to define brought subnets and a VPC. For that, various mock calls will have to be introduced which will return the right subnets.

This is, however, easier said than done because the subnet order and CIDR order will be random. So I'm not sure how to mock that nicely. It will expect different cidrs for different subnet ids than the one returned by the mock which will throw things off.

[github-actions]

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