cmd/go/testdata: fix nits in test

Change-Id: I85fa5e672a476098f8711dcbb5b20ea1a3fa630d
Reviewed-on: https://go-review.googlesource.com/18953Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/testdata/src/vend/hello/hello_test.go

@@ -7,6 +7,6 @@ import (
 
 func TestMsgInternal(t *testing.T) {
 	if strings.Msg != "hello, world" {
-		t.Fatal("unexpected msg: %v", strings.Msg)
+		t.Fatalf("unexpected msg: %v", strings.Msg)
 	}
 }

src/cmd/go/testdata/src/vend/hello/hellox_test.go

@@ -7,6 +7,6 @@ import (
 
 func TestMsgExternal(t *testing.T) {
 	if strings.Msg != "hello, world" {
-		t.Fatal("unexpected msg: %v", strings.Msg)
+		t.Fatalf("unexpected msg: %v", strings.Msg)
 	}
 }