Take subscription e2e test through checkout during registration

What does this MR do and why?

We want to have appropriate end-to-end test coverage for all the registration paths. This MR takes the previous registration through subscription flow spec through a successful checkout and finishes the registration flow.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #387405 (closed)