Extract a test library helper for ErrorWriter{}.

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

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 }