test: recognize new gofrontend error message

The new gofrontend message matches other gofrontend error messages,
so adjust the test to accept it.

For golang#27938
For golang#51237

Change-Id: I29b536f83a0cf22b1dbdae9abc2f5f6cf21d522d
Reviewed-on: https://go-review.googlesource.com/c/go/+/416014
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: ianlancetaylor

test/fixedbugs/issue27938.go

@@ -11,13 +11,13 @@
 package p
 
 type _ struct {
-	F sync.Mutex // ERROR "undefined: sync|expected package"
+	F sync.Mutex // ERROR "undefined: sync|expected package|reference to undefined name"
 }
 
 type _ struct {
-	sync.Mutex // ERROR "undefined: sync|expected package"
+	sync.Mutex // ERROR "undefined: sync|expected package|reference to undefined name"
 }
 
 type _ interface {
-	sync.Mutex // ERROR "undefined: sync|expected package|expected signature or type name"
+	sync.Mutex // ERROR "undefined: sync|expected package|expected signature or type name|reference to undefined name"
 }