Extract "@request.env['devise.mapping'] = Devise.mappings[:user]" to a test helper

What does this MR do?

Extracted @request.env['devise.mapping'] = Devise.mappings[:user] and Rails.application.env_config['devise.mapping'] = Devise.mappings[:user] in our tests into the helper method set_devise_mapping

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

no

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 #34549 (closed)