Additional changes for x-pack/test/banners_functional

[LeeDr]

Summary

ref: #135783
This PR was my attempt at fixing the tests in #135783. It looks like the x-pack/test/banners_functional tests haven't ever been running in CI. They are currently in the list of configs that don't run here https://github.com/elastic/kibana/blob/main/.buildkite/ftr_configs.yml#L52

When I tried to run them locally I hit several problems;

1. the setAdvancedSettingsTextArea in settings_page wasn't working. The save button didn't appear. In this PR I've added another setFocus and send the return key and that seems to get the save button to appear. But it was still flaky so I changed the test to set the banner via the ui settings API
2. The test was setting a custom route, but I couldn't see any reason for that.
3. if it's creating the space via the api, I don't think it even needs the kbn_archive
4. the last test indicates the space selection page would show a default banner (the same as a new space would get) but in reality, Kibana shows the banner from the default space.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
See more potential risk examples

For maintainers

• This was checked for breaking API changes and was labeled appropriately

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