goprotobuf: Use local imports for testdata package.

TBR=r
CC=golang-dev
http://codereview.appspot.com/5832043
diff --git a/proto/text_test.go b/proto/text_test.go
index 4e76f78..e720bdd 100644
--- a/proto/text_test.go
+++ b/proto/text_test.go
@@ -38,7 +38,7 @@
 
 	"code.google.com/p/goprotobuf/proto"
 
-	pb "code.google.com/p/goprotobuf/proto/testdata"
+	pb "./testdata"
 )
 
 func newTestMessage() *pb.MyMessage {