Refactor visibility_level_checker spec

What does this MR do?

While investigating another issue to see how admin users were impacted by Project visibility, I noticed that we weren't testing admin users. As I was adding it, I spotted a few other things to tidy up the file, and squashed it all down into one commit.

• add named subject
• remove unused let declaration
• extract common result variable to hold action response
• add check for admin user

Screenshots

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