Drop main module dependency on test module

I suppose we could solve this other ways, but this utility was
only used in one place right now, so it is easiest to copy it over.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

test/library/ioutil.go

@@ -1,17 +0,0 @@
-/*
-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 }