plugins: adds new no-op plugin that may be used as a test-double

Plugin can be configured to record all inputs and to respond with
arbitrary stdout or error message.  Will support upcoming integration
testing.

build

@@ -20,7 +20,7 @@ echo "Building reference CLI"
 go install "$@" ${REPO_PATH}/cnitool
 
 echo "Building plugins"
-PLUGINS="plugins/meta/* plugins/main/* plugins/ipam/*"
+PLUGINS="plugins/meta/* plugins/main/* plugins/ipam/* plugins/test/*"
 for d in $PLUGINS; do
 	if [ -d $d ]; then
 		plugin=$(basename $d)