strconv: remove use of DeepEqual for testing errors

Comparing errors using DeepEqual breaks if frame information
is added as proposed in Issue #29934.

Updates #29934.

Change-Id: I0372883288f974998138f95f6c7c79a60f922a3e
Reviewed-on: https://go-review.googlesource.com/c/162177


Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>