test: match gofrontend error messages

fixedbugs/bug487.go:17:17: error: function result count mismatch
fixedbugs/bug487.go:18:16: error: function result count mismatch

fixedbugs/issue6977.go:37:26: error: duplicate method ‘m’
fixedbugs/issue6977.go:38:21: error: duplicate method ‘m’
fixedbugs/issue6977.go:39:26: error: duplicate method ‘m’
fixedbugs/issue6977.go:40:21: error: duplicate method ‘m’

Change-Id: Ie3c8a4650cd8f4c239bdceac25dc188a6a50ca34
Reviewed-on: https://go-review.googlesource.com/c/go/+/275178


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>