encoding/json: 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: Ib430c9ddbe588dd1dd51314c408c74c07285e1ff
Reviewed-on: https://go-review.googlesource.com/c/162179


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>