bpo-34040, multiprocessing: Fix test_forkserver_sigkill() (GH-8081)

Fix test_forkserver_sigkill() of test_multiprocessing_forkserver:
give more time to the first child process to complete, double the
sleep in the parent process.

Reduce also the child process sleep from 1000 ms to 500 ms, to not change
the total duration of the test.