[3.12] gh-120039: Reduce expected timeout in test_siginterrupt_off (GH-120047) (#120061)

The process is expected to time out. In the refleak builds,
`support.SHORT_TIMEOUT` is often five minutes and we run the tests six
times, so test_signal was taking >30 minutes.
(cherry picked from commit d419d468ff4aaf6bc673354d0ee41b273d09dd3f)

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