[3.12] Fix test_generators: save/restore warnings filters (GH-108246) (#108249)

Fix test_generators: save/restore warnings filters (GH-108246)

Previously, depending on existing filters, the test
could modify the warnings and so fail as "env changed".
(cherry picked from commit 531930f47f6b2a548d31e62cb4ad3e215a24bf53)

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