[3.11] gh-111929: Fix regrtest --pgo: test_str => test_unicode (GH-111938) (#111940)

gh-111929: Fix regrtest --pgo: test_str => test_unicode (GH-111938)

test_unicode was renamed to test_str in Python 3.13, but Python 3.12
still uses test_unicode name.
(cherry picked from commit 5f42a2bc4017f2ed023f9cf19fdbffabd57527f5)

Co-authored-by: Victor Stinner <vstinner@python.org>