Domains: Add e2e tests for /start/domain (domain-only) flow

[leonardost]

Part of DOMAINS-1636

Proposed Changes

This PR adds two test cases for the /start/domain flow. The tests are:

• Purchase a domain - only buy a domain
• Purchase a domain - create a new site, purchase a plan

This PR also adds two more helper methods, SignupPickPlanPage.selectPlanWithoutSiteCreation and UserSignupPage.signupWithEmailAndWaitForSiteCreation, to account for the different order of steps in the domain-only flow (login happens after plan selection, and site creation happens after login). I'm not too happy with them, but I couldn't think of a more elegant solution right now 😕

Why are these changes being made?

We want to be confident that the domain-only flow won't easily break or have regressions.

Testing Instructions

• Code inspection
• Ensure pre-release e2e tests are passing

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-160252

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-160252&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-160252&env=a8c-for-agencies