[3.14] gh-140601: Refactor ElementTree.iterparse() tests (GH-141499) (GH-141502)

Split existing tests on smaller methods and move them to separate class.
Rename variable "content" to "it".
Use BytesIO instead of StringIO.
Add few more tests.
(cherry picked from commit 2fbd39666663cb5ca1c0e3021ce2e7bc72331020)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>