Add a createRevision helper method in the revision controller test (#667)
Add a createRevision helper method in the revision controller test (#667)

* Add a createRevision helper method in the revision controller test

This refactors some code that is seen everywhere in the test into its
own helper method to reduce duplicated code and make it easier to
understand.

The createRevision method creates a revision in the elaClient, adds it
to the elaInformer, and runs a sync on the controller.

* responding to review comments

knative/serving,knative/serving