[3.12] gh-115133: Fix tests for XMLPullParser with Expat 2.6.0 (GH-115164) (GH-115288)

Feeding the parser by too small chunks defers parsing to prevent
CVE-2023-52425. Future versions of Expat may be more reactive.
(cherry picked from commit 4a08e7b3431cd32a0daf22a33421cd3035343dc4)

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