tests: add simple e2e test for guestbook

[justinsb]

This highlights a few issues (out of date images, etc.),
and will allow us to fix them with test coverage.

[kubernetes-prow]

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[soltysh]

@justinsb any particular reason for a gh workflow, rather than just hooking this up to prow?

[justinsb]

@justinsb any particular reason for a gh workflow, rather than just hooking this up to prow?

I was just being lazy. Sent kubernetes/test-infra#37422 to add some prow.

Created a single entrypoint though, rather than having to go back and forth to prow config e.g. if we add a test for guestbook-go. Should also let us share a cluster between examples, which should be slightly faster.