gh-35466: refactor poset examples for better code coverage
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### :books: Description
This introduces in the file `poset_examples` an auxiliary function to
check the input.
This should help to increase the code coverage there.
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URL: https://github.com/sagemath/sage/pull/35466
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
