xerrors: avoid infinite loop in test (detailed).Error method Fixes golang/go#40536 Change-Id: I5daaec48301cdedb2efcfadaf1dcd41cb8e96b01 Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/246640 Run-TryBot: Bryan C. Mills <bcmills@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Alexander Rakoczy <alex@golang.org>