Use presubmit-tests.sh helper from prow-tests image (#261)
Use presubmit-tests.sh helper from prow-tests image (#261)

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 https://github.com/knative/test-infra/pull/12
knative/build,knative/build