[3.13] gh-118846: Fix PGO tests in free-threaded build (GH-118862) (#118867)

Avoid immortalizing objects in tests that verify garbage collection of
classes or modules.

This fixes test_ordered_dict and test_struct.
(cherry picked from commit 1b1db2fd9a531e26b79b34667bccfb938c4d184d)

Co-authored-by: Sam Gross <colesbury@gmail.com>