Some doctests fail to recognize synonyms of the expected answers (meta-ticket)

[EmmanuelCharpentier]

Steps To Reproduce

For several beta releases, I noticed and reported that some doctests fail to recognize a synonym of the expected answer, and therefore raise an undeserved failure.

Hunting for such cases is not obvious. The present ticket aims at collecting such cases and pointing to

Expected Behavior

An example :

charpent@brique:/usr/local/sage-10$ src/bin/sage -t --long --warn-long 30.0 --random-seed=15939232666016095847464046535027297743 src/sage/rings/polynomial/multi_polynomial_ideal.py 

[ Elided... ]

----------------------------------------------------------------------
src/bin/sage -t --long --warn-long 30.0 --random-seed=15939232666016095847464046535027297743 src/sage/rings/polynomial/multi_polynomial_ideal.py  # 1 doctest failed
----------------------------------------------------------------------
Total time for all tests: 12.0 seconds
    cpu time: 6.9 seconds
    cumulative wall time: 6.8 seconds
Features detected for doctesting: msolve,sage.combinat,sage.libs.flint,sage.libs.giac,sage.libs.singular,sage.modules,sage.plot,sage.rings.finite_rings,sage.rings.number_field,sage.symbolic

### Actual Behavior

The same example : 

charpent@brique:/usr/local/sage-10$ src/bin/sage -t --long --warn-long 30.0 --random-seed=15939232666016095847464046535027297743 src/sage/rings/polynomial/multi_polynomial_ideal.py

[ Elided... ]

---

All tests passed!

### Additional Information

Failures of this type will be reported here, and fixed in specific tickets.

### Environment

- **OS**:  Debian testing
- **Sage Version**: 10.8.beta9


### Checklist

- [x] I have searched the existing issues for a bug report that matches the one I want to file, without success.
- [x] I have read the documentation and troubleshoot guide

[EmmanuelCharpentier]

Ticket for multi_polynomial_ideal : #41193