[WIP] Add e2e tests for kubectl plugin

[alexkursell]

What this PR does / why we need it: This PR adds e2e tests for the ingress-nginx kubectl plugin. It also adds tests for the commands added by #3870, so this should not be merged until that is.

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[ElvinEfendi]

@alexkursell would it be easier to stick to unit tests only for the plugin?

[alexkursell]

@alexkursell would it be easier to stick to unit tests only for the plugin?

Yes. e2e testing this is kind of painful, and I don't really see that much benefit to it. I'll see what in the plugin can be unit tested. Closing this for now.