WIP: Improved signup_spec to run in a state that is actually possible to use

What does this MR do?

CE port of: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1839

Fixes signup_spec.

Our sign up form cannot be used for registration when JS is disabled so I'm not sure how it was even getting to the failures it gets to on CI. I could get this to pass consistently locally with JS enabled, so I fixed it up and hopefully it will be more stable when we test it in a predictable way.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2362