test: added test case in prefer_const_border_radius rule

[vlkonoshenko]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 426

• Total lines of source code: 7455

• Total classes: 291

• Average Cyclomatic Number per line of code: 0.35

• Average Source Lines of Code per method: 6

• Total tech debt: 1310.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #876 (5ace123) into master (169f6ae) will not change coverage.
The diff coverage is n/a.

❗ Current head 5ace123 differs from pull request most recent head 58e19a6. Consider uploading reports for the commit 58e19a6 to get more accurate results

@@           Coverage Diff           @@
##           master     #876   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files         280      280           
  Lines        5942     5942           
=======================================
  Hits         5186     5186           
  Misses        756      756           

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