Refactor the EE::RegistrationsController spec

What does this MR do?

This MR is a follow-up to work done in !40304 (merged), as mentioned in this comment & this comment.

While updating & adding to the tests for this controller it felt like the current implementation was difficult to reason about and unlikely to scale much further. This change uses more lines of code to create fewer tests but I believe that we still maintain adequate test coverage and the expressiveness of the context and describe blocks makes it easier to see what’s really being tested.

No changelog is necessary as this is a purely developer-facing change. Likewise, this change does not alter anything which requires updated documentation.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Depends on, and was born out of, !40304 (merged)