GKE self managed setup script should have an e2e test

[KatrinaHoffert]

Note: should be viewed as dependent #759.

The script was broken already, so there's certainly merit in a test to avoid that. However, it's difficult because the test will be slow and require many things:

1. Script requires cluster ownership
2. Ideally should have push access to some container registry
3. Requires an existing cluster (so likely have to set one up or at least be dependent on another e2e test)
4. Slow because it needs to restart a cluster and realistically, a test would also need to create an ingress, which takes a few minutes and uses GCP resources.

Things that could be done to make this task easier:

1. Commands that we use to populate the gce.conf file could be split into their own functions and tested on their own, then the larger test can just use the CLI flags to override em.
2. We could just check the output files we created and the commands we run. Fake the gcloud, kubectl, and make and just test what they execute. Would make it a unit test instead of an e2e test, but at least would catch changes and would be much faster to run.
3. We could use --dry-run with kubectl to at least confirm our YAML is valid (since that's one thing that was broken before!).
4. We could really avoid making any ingress resources. Just checking that the controller pod starts without errors would probably be sufficient combined with other tests. Just risky due to the fact the configuration flow is not the same as those tests.

[KatrinaHoffert]

/good-first-issue
/kind feature
/cc KatrinaHoffert

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