Explanation copied from GoogleCloudPlatform/compute-image-tools#472:
I've noticed, for this test workflow that when one SubWorkflow
ends, it forces ending all of the others subworkflows, ignoring their tests:
[test-subworkflow.quick]: 2018-07-02T21:01:46-03:00 Running step "wait-forever" (WaitForInstancesSignal)
[test-subworkflow.quick.wait-forever]: 2018-07-02T21:01:46-03:00 WaitForInstancesSignal: Instance "inst-quick-test-test-subworkflow-quick-14v47": watching serial port 1, SuccessMatch: "BOOTED".
[test-subworkflow.loop]: 2018-07-02T21:01:48-03:00 Running step "wait-forever" (WaitForInstancesSignal)
[test-subworkflow.loop.wait-forever]: 2018-07-02T21:01:48-03:00 WaitForInstancesSignal: Instance "inst-loop-test-test-subworkflow-loop-8jrgh": watching serial port 1, SuccessMatch: "b32531e7ca631a108a8d924262584a5e", StatusMatch: "Printing".