gh-37561: enrich the test_karatsuba failure message with explicit elements
To help debug the random failure in `test_karatsuba_multiplication`, let
us have a more precise error message in case of failure there.
cf #35715
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/37561
Reported by: Frédéric Chapoton
Reviewer(s): Martin Rubey
