Add feature spec for Continuous Onboarding iteration 2 experiment

As part of the GitLab experimentation guidelines, it states that:

Experiments must have corresponding frontend or feature tests to ensure they exist in the application.

That has not yet been done for the change_continuous_onboarding_link_urls experiment.

In order to fulfill that requirement, we should add a simple, happy-path feature spec which ensures that our experiment is present as part of the flow we expect it to be present in.