tests: Improve LtiAdvantageHelper
tests: Improve LtiAdvantageHelper

We added a FactoryBot generator for client IDs. It'd be better to use that
than generating one manually. Less work and more consistency.

atomicjolt/lti_starter_app,atomicjolt/adhesion,atomicjolt/adhesion,atomicjolt/lti_starter_app,atomicjolt/lti_starter_app,atomicjolt/adhesion,atomicjolt/adhesion,atomicjolt/lti_starter_app