Add overlay tests

[caugustus-sourcegraph]

Creates a new test suite based off the existing restricted tests and expands it to include other overlays. The suite creates a new GKE cluster (latest version) and reuses it for each overlay. My goal here was to stay close to the original test implementation and see where the sticking points were before changing too dramatically.

Not all overlays are tested. This is not yet hooked into CI because it's still a proof-of-concept and needs additional work. I've been successfully using it to verify local changes and it's even revealed a potential problem with the storage overlay.

Checklist

• [ ] CHANGELOG.md updated
• [ ] K8s Upgrade notes updated
• [ ] Sister deploy-sourcegraph-docker change:
• [ ] All images have a valid tag and SHA256 sum