test/fixedbugs/bug295.go: fix test in anticipation of future gc fix

See also issue 6428.

R=r, rsc
CC=golang-dev
https://golang.org/cl/13794043

test/fixedbugs/bug295.go

@@ -6,7 +6,9 @@
 
 package main
 
-import . "testing"  // defines top-level T
+import . "testing"  // defines file-level T
+
+type _ B // make use of package "testing" (but don't refer to T)
 
 type S struct {
 	T int