[3.12] gh-113205: test_multiprocessing.test_terminate: Shorter sleep for threadpools (GH-114186) (GH-114222)

Threads can't be forced to terminate (without potentially corrupting too much
state), so the  expected behaviour of `ThreadPool.terminate` is to wait for
the currently executing tasks to finish.

Use shorter sleep time for threadpools, so if a task manages to start, the test
doesn't block for long.

(cherry picked from commit c1db9606081bdbe0207f83a861a3c70c356d3704)

Co-authored-by: Petr Viktorin <encukou@gmail.com>