bpo-37123: multiprocessing test_mymanager() accepts SIGTERM (GH-16349)

Multiprocessing test test_mymanager() now also expects -SIGTERM, not
only exitcode 0.

bpo-30356: BaseManager._finalize_manager() sends SIGTERM to the
manager process if it takes longer than 1 second to stop, which
happens on slow buildbots.