gh-38763: use StructureDescription() in gap/element.pyx tests
This will fix #38760
The output of these tests is too verbose, and this unstable. We fix it
here
### :memo: Checklist
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preview.
URL: https://github.com/sagemath/sage/pull/38763
Reported by: Dima Pasechnik
Reviewer(s): Martin Rubey
