Test against a cluster in CI

This adds a test-integration which tests the kubectl, helm and tiller against a k3s cluster. It may not be totally worthwhile but this was prompted by @proglottis mentioning another MR was failing to have helm and tiller communicating properly !5 (comment 165968877) . So it made me think about how we could have an integration test.

In this current form I'm not sure if this will be valuable enough but if it's reliable maybe we can keep it. Right now it is really just testing a helm, tiller, kubectl and k3s and not really testing our own code very much but it occurs to me this is a good proof of concept we may want to use in other CI jobs. In particular when we have https://gitlab.com/gitlab-org/cluster-integration/auto-deploy-image/ we may also want to test it out like this so we can test our deploy code against a K8s cluster.

This integration test also only adds 25s to our pipeline time seeing as it's 50s and runs parallel to the other test job which takes 25s.