refactor: Avoid importing string literals in tests

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What does this MR do and why?

• When working on another MR, I noticed some potential for improving this test
• Apply testing best practice to avoid importing copy strings in specs, see https://docs.gitlab.com/ee/development/i18n/externalization.html#recommendations

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Only changes related to tests

How to set up and validate locally

1. Execute the tests

yarn jest spec/frontend/projects/settings/repository/branch_rules/components/branch_rule_spec.js