test: Match gccgo error messages.

Another case where gccgo and gc report similar but not
identical errors for a recursive interface.

bug251.go:11: error: invalid recursive interface

R=rsc
CC=golang-dev
https://golang.org/cl/2094041

test/fixedbugs/bug251.go

@@ -8,14 +8,14 @@ package main
 
 type I1 interface {
 	m() I2
-	I2
+	I2 // GCCGO_ERROR "loop|interface"
 }
 
 type I2 interface {
-	I1 // ERROR "loop|interface"
+	I1 // GC_ERROR "loop|interface"
 }
 
 
-var i1 I1 = i2 // ERROR "missing m method|need type assertion"
+var i1 I1 = i2 // GC_ERROR "missing m method|need type assertion"
 var i2 I2
 var i2a I2 = i1