[3.10] gh-100933: Improve `check_element` helper in `test_xml_etree` (GH-100934) (#101687)

Items checked by this test are always `str` and `dict` instances.
(cherry picked from commit eb49d32b9af0b3b01a5588626179187f11d145c9)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>