It's a little tricky to find a balance between "brittle" and "thorough"
in this test--I'd like to verify that e.g. the certificate is in
clusters[0].cluster.certificate-authority-data, not at the root. On the
other hand, we can't actually show that it's a valid kubeconfig file
without actually *using* it, so there's a hard upper limit on the
strength of the assertions. I've settled on verifying that all the
settings make it into the file and the file is syntactically-valid yaml.
The recommended way to test code that uses exec.Cmd involves setting up
a real exec.Cmd that invokes `go test` with additional arguments that
fire off a specially-constructed test that behaves the way the mocked-
out script would be expected to do. It's a sensible way to test exec.Cmd
itself, but for code that merely invokes it, I think it makes more sense
to use actual mocks.
