Run open interface system tests with both window behaviours

Description of work

Test interface opening with both kinds of additional window behaviour.

Fixes #31303

To test:

Code review
Tests should pass

Reviewer

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Code Review

Is the code of an acceptable quality?
Does the code conform to the coding standards?
Are the unit tests small and test the class in isolation?
If there is GUI work does it follow the GUI standards?
If there are changes in the release notes then do they describe the changes appropriately?
Do the release notes conform to the release notes guide?

Functional Tests

Do changes function as described? Add comments below that describe the tests performed?
Do the changes handle unexpected situations, e.g. bad input?
Has the relevant (user and developer) documentation been added/updated?

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Gatekeeper

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mantidproject / mantid