Fix or document issue for test failures when cni is a submodule

[rosenhouse]

This is to track the following issue: When this repo is a submodule of another repo, and then you run ./test, you'll see:

GetVersion, integration tests
/go/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/pkg/invoke/get_version_integration_test.go:78
  correctly reporting plugin versions
  /go/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/vendor/github.com/onsi/ginkgo/extensions/table/table.go:96
    historical: when plugins started reporting their own version list [It]
    /go/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/vendor/github.com/onsi/ginkgo/extensions/table/table_entry.go:37

    Expected success, but got an error:
        <*errors.errorString | 0xc82047ef90>: {
            s: "unable to find cni repo root, landed at \"/go/src/github.com/containernetworking/cni/gopath/src\"",
        }
        unable to find cni repo root, landed at "/go/src/github.com/containernetworking/cni/gopath/src"

    /go/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/pkg/invoke/get_version_integration_test.go:49

I'd like to either document this failure and how to avoid it, or better yet fix it, so that the tests run cleanly.

[rosenhouse]

Confirmed, still an issue :-(