K8s tutorial: add integration tests for COS Lite relations

[dwilding]

Per Tony's comment, we should expand Observe your charm with COS Lite to include integration tests for the relations added in the chapter.

This could be done before or after switching to Jubilant (which is planned as part of #1749).

[benhoyt]

This could be pretty light, even just 1 test to show the pattern.