TestStepHandler wait for processes to exit
Summary: This test formerly exited once a STEP_SUCCESS event was emitted for every step. However there is a gap between when a step worker emits that event, and when the step worker's process exits. This caused flakes on windows, where processes cannot still be alive when a test exits.
Test Plan: existing test
Reviewers: dgibson, max
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8081