[3.13] gh-116622: Switch test_stress_delivery_simultaneous from SIGUSR1 to SIGUSR2 (GH-123981) (#123988)

gh-116622: Switch test_stress_delivery_simultaneous from SIGUSR1 to SIGUSR2 (GH-123981)

Use SIGUSR1 instead of SIGUSR2 to improve reliability of signal stress test on Android.
(cherry picked from commit 43303e362e3a7e2d96747d881021a14c7f7e3d0b)

Co-authored-by: Malcolm Smith <smith@chaquo.com>