[Guided onboarding] Add API integration tests

[alisonelizabeth]

This issue is to track adding API integration tests for the guided onboarding routes (defined here).

Proposed test cases:

• GET /state

  • Returns the state for all guides (?active=false)
  • Returns state for the active guide (?active=true)
  • Returns state from saved object if it exists
  • Returns an empty array if saved object doesn't exist

• PUT /state

  • Updates a guide that has an existing saved object
  • Updates a guide that does not have a saved object
  • Updates any existing active guides to inactive

[elasticmachine]

Pinging @elastic/platform-onboarding (Team:Journey/Onboarding)