E2E test: emit events from a utility pod

[grantr]

Currently the E2E tests use (roughly) this procedure:

1. Create a KubernetesEventSource
2. Wait for it to become ready
3. Create a Pod
4. Wait for it to start
5. Wait for a Created container string in the message-dumper logs
6. Wait for a `Started containe string in the message-dumper logs

This is a great test of KubernetesEventSource, but IMO not a great test of eventing core. Any failure in the source also fails the test, and source implementations are still young and relatively untested. This has led to hard-to-diagnose failures caused by the dependency on eventing-sources (see #683).

Instead, I suggest using the sendevent helper to publish messages in tests. The E2E tests can run this as a Pod to get a message into the channel under test. In addition to likely being more reliable and predictable than a source, this gives the test control over the content, timing, and number of messages, allowing for more complex tests that exercise message routing across multiple channels and subscriptions.

I also suggest moving or copying the message_dumper utility from https://github.com/knative/eventing-sources/tree/master/cmd to this repo alongside sendevent.

[vaikas]

I think this is a great idea. I would like to test a subscription that takes in an event and sends it to another function via channel also as playing with that has proved that there are some short comings or that it's not quite as easy as it should.

[n3wscott]

/assign