Create subscription e2e test all the way through checkout during registration

The following discussion from !108354 (merged) should be addressed:

• @dstull started a discussion:

  We'll stop here for now and fire up an issue to take this all the way through checkout to see where it lands us after checkout as well.

Plan 🚧

This will be an evolving plan as we'll have to see how many permutations exist and will need to stub out customersDot calls along the way.

We'll start with ee/spec/features/registrations/saas/subscription_flow_paid_plan_spec.rb and take it through to the next steps to see how many permutations exist from a successful checkout