To actually use CNI plugins in the given CNI_PATH, we need to add
CNI_PATH to PATH because CmdAddWithResult() does this:
os.Setenv("CNI_PATH", os.Getenv("PATH"))
Go's "..." syntax (eg, ./plugins/...) doesn't traverse symlinks, so
go test wasn't finding the vendor/ directory for imports. To get around
that we have to specify each testable package specifically rather
than use "...".
