testing/iotest: add ErrReader

[caarlos0]

Abstract

Add a iotest.ErrReader io.Reader implementation that reads 0 bytes and fails.

This would be useful for quickly adding test cases for when Read doesn't work for some reason.

Background

The iotest package already have a couple of other io.Reader implementations, but we don't have any that fails right away.

On nearly all codebases I worked in, I created this same failReader/errReader implementation. Judging by this and this github searches, I'm not alone.

Proposal

Add it to the standard library, under the iotest package.

I actually have the PR opened already: #34741

Rationale

Make it easier to add test cases for read failures.

[rsc]

I'd call it ErrReader, to match DataErrReader, but this seems fine to me.
That is:

package iotest

// ErrReader returns an io.Reader that returns 0, err from all Read calls.
func ErrReader(err error) io.Reader

[caarlos0]

ErrReader sounds good to me as well. Updated the proposal.

EDIT: updated PR as well.

[rsc]

Adding to proposal minutes. Seems fine to me, as I said before.

[rsc]

Based on how trivial this is and the lack of any objections, this seems like a likely accept.

[rsc]

No change in consensus, so accepted.