pkg/invoke: refactor plugin exec and backfill unit tests

invoke/fakes/cni_args.go

@@ -0,0 +1,27 @@
+// Copyright 2016 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fakes
+
+type CNIArgs struct {
+	AsEnvCall struct {
+		Returns struct {
+			Env []string
+		}
+	}
+}
+
+func (a *CNIArgs) AsEnv() []string {
+	return a.AsEnvCall.Returns.Env
+}