testing/iotest: add TestReader to test readers

testing/iotest: add TestReader to test readers

There are many reader behaviors that are subtle and
worth testing, and it's nice to have one complete tester
instead of many incomplete ones.

For #41190, which will use this as part of a larger
file system implementation tester.

Change-Id: Ib4cc7fae94b0d9b45dfacadc52baa77ad3761322
Reviewed-on: https://go-review.googlesource.com/c/go/+/243909
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>