Improve seat type calculator root namespace variable naming

What does this MR do and why?

Using a subgroup instead of a group for testing the seat type calculator to ensure the seat type calculator is using logic that always considers the root ancestor of a namespace and not only the namespace itself. The root ancestor of a root-level group is the group itself.

See also the discussion here.

References

• Resolves #563373 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Green test suite.

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