Setup PR testing for build-pipeline unit tests

[bobcatfish]

Expected Behavior

Every PR should have unit tests run against it automatically.

(The long term dream is that we use the Pipeline CRD reference implementation itself to test this repo, but since it is just an API right now, that's a ways off!)

Actual Behavior

We have no automation! Is master broken? WHO KNOWS IT'S A CRAZY WORLD 🤠

Steps to Reproduce the Problem

1. Open a PR with broken unit tests
2. Notice that nothing catches this

To actually run the tests:

make test

(see also the DEVELOPMENT.md)

Additional Info

We would be fine with using something like Travis initially, but it would have to be enabled for the entire knative org, and since the rest of the org is using Prow, we can probably jump on that train.

Configuration for Prow for the other knative projects lives in https://github.com/knative/test-infra/tree/master/ci/prow

[bobcatfish]

Quick notes re. Prow setup:

• I think this config.yaml is the prow config for knative, but maybe we'll need to add something to other files in the prow dir as well?
• We probably need to add something to the gubernator config so that we can view logs

Later on we'll want to add some testgrid config too probably but we don't need that right away.

Docs:

• Prow docs
• Configuring a prow job, Adding new prow jobs

Folks who are great resources for questions, review etc. would be @adrcunha @cjwagner

[adrcunha]

• See https://github.com/knative/test-infra/blob/master/ci/prow/prow_setup.md%23setting-up-prow-for-a-new-repo for more details on setting up the Prow jobs.
• Travis is not a supported CI/CD for Knative.
• Kubernetes Prow docs are a nice reference, but since Knative has his own Prow instance, not all steps apply.

[bobcatfish]

Whoa i dunno how this stayed open so long, this was done ages ago :D