Integration testing

[cpuguy83]

There is a simple e2e test that executes the copa CLI.

I think we'd benefit greatly from more integration level tests that can just execute go code and inspect outputs. Probably this means some refactoring where instead of taking a plain buildkit.Client we need to take a gateway client.

We basically do a lot of this in https://github.com/Azure/dalec. https://pkg.go.dev/github.com/Azure/dalec/test/testenv?utm_source=godoc#BuildxEnv.RunTest

By utilizing the gateway client we can write tests that just look at the state of the content before its been exported to the client (just like as if you were code running inside copa itself). One thing it cannot do is look at individual layers, so it'd still need to export that content out to have tests to check things like the number of layers are what is expected.

[github-actions[bot]]

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[leodewang]

@robert-cronin