flaky test: test_start_on_file

[d4l3k]

There's been a number of failures for test_start_on_file in the CI that have popped up over time. We should fix this test so it's more stable.

This failure was on macOS but not sure if it's specific or an issue in general.
Ex: https://github.com/pytorch/torchx/runs/5204752446?check_suite_focus=true

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FAIL: test_start_on_file (torchx.apps.utils.test.process_monitor_test.ProcessTest)
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SystemExit: -15

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/runner/work/torchx/torchx/torchx/apps/utils/test/process_monitor_test.py", line 108, in test_start_on_file
    main(args)
AssertionError: "^0$" does not match "-15"

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Meta:
We don't see failures internally though stress tests don't seem to be enabled. https://www.internalfb.com/intern/test/281475024185650?ref_report_id=0