test: recognize new gofrontend error message

test: recognize new gofrontend error message

As of https://golang.org/cl/273886:

fixedbugs/bug340.go:15:18: error: reference to method ‘x’ in interface with no methods

For golang/go#10700

Change-Id: Id29eb0e34bbb524117614229c4c27cfd17dae286
Reviewed-on: https://go-review.googlesource.com/c/go/+/273887
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>