test: make tests actually work when packages have vendored imports

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 "...".

build

@@ -2,7 +2,7 @@
 set -e
 
 ORG_PATH="github.com/containernetworking"
-REPO_PATH="${ORG_PATH}/plugins"
+export REPO_PATH="${ORG_PATH}/plugins"
 
 if [ ! -h gopath/src/${REPO_PATH} ]; then
 	mkdir -p gopath/src/${ORG_PATH}