Incomplete test coverage for `unittest.util`

[tomasr8]

Bug report

Bug description:

safe_repr, sorted_list_difference and unorderable_list_difference inside unittest.util are not properly tested. I'll send a PR shortly with some tests ;)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-121708: Improve test coverage for unittest.util #121713
• [3.12] gh-121708: Improve test coverage for unittest.util (GH-121713) #121737
• [3.13] gh-121708: Improve test coverage for unittest.util (GH-121713) #121738

[encukou]

Thank you!