test: match gofrontend error messages

fixedbugs/issue11614.go:14:9: error: interface contains embedded non-interface
fixedbugs/issue11614.go:22:20: error: interface contains embedded non-interface

Change-Id: Ie9875916697833f5fa28ab890218851a741120ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/278175


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