pythongh-140601: Refactor ElementTree.iterparse() tests (pythonGH-141499)

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 2fbd396)

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

Author: serhiy-storchaka