errors/fmt: fix tests for new panic message in Go 1.14
errors/fmt: fix tests for new panic message in Go 1.14

Fixes golang/go#30622.

Change-Id: Ia99d06914aeffb219aaf54afe0f66e573a595e63
Reviewed-on: https://go-review.googlesource.com/c/exp/+/204078
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/exp,golang/exp,golang/exp