Common e2e/conformance infrastructure

[mattmoor]

This repo houses shared Knative libraries. The original intent was to share common controller libraries and whatnot, but I think that there's a clear and growing need to share common test and release logic as well.

Two clear candidates off the top of my head (the common bits):

1. github.com/knative/serving/pkg/controller/testing
2. github.com/knative/serving/test

cc @bobcatfish (emeritus)

[mattmoor]

/area test-and-release

[srinivashegde86]

/assign @srinivashegde86