gh-35220: add tests for solved issue about fraction fields
### 📚 Description
Add a simple test for #15971.
Closes #15971.
### 📝 Checklist
- [x] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [x] I have linked an issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/35220
Reported by: Vincent Delecroix
Reviewer(s): Frédéric Chapoton
