It turned out that changes like
https://github.com/kubernetes-csi/csi-lib-utils/pull/33 should better
have been committed after `go mod tidy` because that adds some
indirect dependencies in that example.

The revised `test-vendor` checks for that and (just in case that this
ever becomes desired) allows projects to not have a vendor directory
when using `go mod`.

How to use `go mod` properly gets documented in the README.md, because
there are such pitfalls.