pkg/skel: refactor to use dependency injection

Extract dependencies on os to enable more complete unit test coverage

testutils/bad_reader.go

@@ -0,0 +1,32 @@
+// Copyright 2014 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 testutils
+
+import "errors"
+
+type BadReader struct {
+	Error error
+}
+
+func (r *BadReader) Read(buffer []byte) (int, error) {
+	if r.Error != nil {
+		return 0, r.Error
+	}
+	return 0, errors.New("banana")
+}
+
+func (r *BadReader) Close() error {
+	return nil
+}