gh-113205: test_multiprocessing.test_terminate: Test the API on threadpools (#114186)

gh-113205: test_multiprocessing.test_terminate: Test the API works on threadpools

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.

The entire test was skipped in GH-110848 (0e9c364f4ac18a2237bdbac702b96bcf8ef9cb09).
Instead of skipping it entirely, we should ensure the API eventually succeeds:
use a shorter timeout.

For the record: on my machine, when the test is un-skipped, the task manages to
start in about 1.5% cases.