Add tracking test for change_continuous_onboarding_link_urls experiment

What does this MR do and why?

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.

Related to #344574 (closed)