bpo-30317, test_multiprocessing: fix test_timeout() (GH-7957)

Tolerate a different of 50 ms, instead of just 30 ms, in
test_timeout() of multiprocessing tests. This change should fix such
test failure on Windows:

FAIL: test_timeout (test.test_multiprocessing_spawn.WithProcessesTestQueue)
Traceback (most recent call last):
  File "lib\test\_test_multiprocessing.py", line 753, in test_timeout
    self.assertGreaterEqual(delta, 0.170)
AssertionError: 0.16138982772827148 not greater than or equal to 0.17