Extract a test library helper for ErrorWriter{}.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

cmd/placeholder-name/main_test.go

@@ -16,12 +16,10 @@ import (
 	"github.com/sclevine/spec/report"
 	"github.com/stretchr/testify/require"
 	"k8s.io/client-go/pkg/apis/clientauthentication"
+
+	"github.com/suzerain-io/placeholder-name/test/library"
 )
 
-type errWriter struct{ returnErr error }
-
-func (e *errWriter) Write([]byte) (int, error) { return 0, e.returnErr }
-
 func TestRun(t *testing.T) {
 	spec.Run(t, "Run", func(t *testing.T, when spec.G, it spec.S) {
 		var buffer *bytes.Buffer
@@ -97,7 +95,7 @@ func TestRun(t *testing.T) {
 			})
 
 			it("returns an error", func() {
-				err := run(envGetter, tokenExchanger, &errWriter{returnErr: fmt.Errorf("some IO error")}, 30*time.Second)
+				err := run(envGetter, tokenExchanger, &library.ErrorWriter{ReturnError: fmt.Errorf("some IO error")}, 30*time.Second)
 				require.EqualError(t, err, "failed to marshal response to stdout: some IO error")
 			})
 		}, spec.Parallel())

test/library/ioutil.go

@@ -0,0 +1,17 @@
+/*
+Copyright 2020 VMware, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package library
+
+import "io"
+
+// ErrorWriter implements io.Writer by returning a fixed error.
+type ErrorWriter struct {
+	ReturnError error
+}
+
+var _ io.Writer = &ErrorWriter{}
+
+func (e *ErrorWriter) Write([]byte) (int, error) { return 0, e.ReturnError }