[Telemetry] Add FTRs to validate the expected behavior of the banners

[afharo]

To avoid introducing regressions such as #159870, we should write FTRs that test the functional expected behavior of the banners:

1. When telemetry.optIn: true (or not explicitly specified), show the banner to inform the user.
2. When Opted-In via Advanced settings, do not show the banner anymore (not even for newer versions).
3. When telemetry.optIn: false (on fresh Kibana install), show the banner to inform the user.
4. When opted-out (either via telemetry.optIn: false or via Advanced Settings), and only notified in a previous minor, notify again.
5. No notifications showed when telemetry.banner: false or allowChangingOptInStatus: false.

I didn't introduce these new FTRs when fixing #159870 because, currently, our functional tests have the banners explicitly disabled, and I didn't want to miss the chance to ship the bug to 8.8.2

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[afharo]

I noticed https://github.com/elastic/kibana/blob/main/test/plugin_functional/test_suites/telemetry/telemetry.ts already tests most of it.

We only need to test the dismissal button.