bpo-33913: Fix test_multiprocessing_main_handling (GH-7972)

bpo-30339, bpo-33913:

* Increase timeout from 10 seconds to 1 minute in
  test_source_main_skipped_in_children source of
  test_multiprocessing_main_handling.
* Replace time.time() with time.monotonic().
* On timeout, include the duration in the error message.