[3.12] gh-110647: Fix signal test_stress_modifying_handlers() (GH-110650) (#110658)

gh-110647: Fix signal test_stress_modifying_handlers() (GH-110650)

* cycle_handlers() now waits until at least one signal is received.
* num_received_signals can be equal to num_sent_signals.
(cherry picked from commit e07c37cd5212c9d13749b4d02a1d68e1efcba6cf)

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