[Telemetry] Test helpers for kibana_system

[afharo]

We should provide some test helpers for devs to make sure the data they are collecting is actually accessible by the internal kibana_system.

At the moment, we have the service https://github.com/elastic/kibana/blob/master/x-pack/test/api_integration/services/usage_api.ts that folks can use to retrieve the telemetry in their FTR. We could improve this service by making it more accessible (not only in the X-Pack API Integration tests), and by performing the calls with the right kibana_system user to make sure they can test accordingly.

Then add some documentation and examples encouraging this kind of tests.

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-telemetry (Team:KibanaTelemetry)

[afharo]

We could have caught this bug (#94980) earlier if we had these helpers implemented 😢

[afharo]

If #96538 is to be implemented, we might not need to do this.