Add config and action integration tests to K8s tutorial charms

[dwilding]

These chapters of the K8s tutorial don't show how to write integration tests:

• Make your charm configurable
• Expose operational tasks via actions

@tonyandrewmeyer and I discussed this today. We don't think it's valuable to extend the time it takes to complete the tutorial with another two rounds of running integration tests. So here's our plan:

1. Add integration tests to the relevant example charms. (Probably a good idea to comment that the tests go beyond the tutorial.)

2. In the tutorial chapters, mention that adding integration tests is an exercise for the reader. They can refer to How to write integration tests for a charm, which has clearly-named sections that should help. If the reader is stuck, they can inspect the example charms.