Docs: explain how to run/debug e2e tests (#529)
Docs: explain how to run/debug e2e tests (#529)

* docs: explain how to run tests

* first draft

* add link to knative eventing tests

* refactor some sections

* update note for credentials

* fix lint error

* fix trailing whitespace
knative-sandbox/eventing-rabbitmq,knative-sandbox/eventing-rabbitmq