Use presubmit-tests.sh helper from prow-tests image (#1628)
Use presubmit-tests.sh helper from prow-tests image (#1628)

We're consolidating the test infrastructure into a single place, so all repos get the same fixes, updates and new features.

presubmit-tests.sh helper is implemented by knative/test-infra#12
knative/serving,knative/serving