Support packaging go test binaries

The ability to package the output of go test -c would be quite handy.

My use case here is for integration tests that get built from the same source as the binary, and the resultant image is used in a post-deployment verification phase (e.g. soon to be released skaffold verify capabilities).

I bet you could do something like this with crane append and crane mutate --entrypoint as described in https://github.com/google/go-containerregistry/blob/main/cmd/crane/recipes.md#bundle-directory-contents-into-an-image

Related: https://github.com/google/ko/issues/96

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