Use the correct module path in (make test-skopeo)

[mtrmac]

The current go mod edit c/image=... did not affect the actually used version at all.

[mtrmac]

This has extra commits that break ABI to verify that the tests actually detect that; I’ll remove them if the output looks right.

[mtrmac]

Looks reasonable:

+ pwd
+ project_path=/gopath/src/github.com/containers/image
+ GO111MODULE=on go list .
+ project_module=github.com/containers/image/v4
+ mktemp -d
+ export GOPATH=/tmp/tmp.r65LKzqzAt
+ skopeo_path=/tmp/tmp.r65LKzqzAt/src/github.com/containers/skopeo
+ git clone -b master https://github.com/containers/skopeo /tmp/tmp.r65LKzqzAt/src/github.com/containers/skopeo
+ cd /tmp/tmp.r65LKzqzAt/src/github.com/containers/skopeo
+ GO111MODULE=on go mod edit -replace github.com/containers/image/v4=/gopath/src/github.com/containers/image
+ make vendor
+ make BUILDTAGS=btrfs_noversion libdm_no_deferred_remove binary-local test-all-local
…
cmd/skopeo/signing.go:53:20: undefined: signature.SignDockerManifest

fails as expected.

[mtrmac]

@vrothberg FYI, merging this now without review to minimize the risk of PRs that break Skopeo without us noticing sneaking in.

PTAL to verify I’m not doing something idiotic when you get a chance.

[mtrmac]

NOTE: Looking at Travis logs, it does (git fetch $the_pr && git checkout FETCH_HEAD), i.e. this PR only affects other PRs that have been rebased on top. Re-running travis tests that have been branched off earlier won’t work, unless that changes the checkout process.

[vrothberg]

Nice trick 👍 LGTM