exp/winfsnotify: remove reference to _test

Updates #2573.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/5677063

src/pkg/exp/winfsnotify/winfsnotify_test.go

@@ -115,7 +115,13 @@ func TestNotifyClose(t *testing.T) {
 		t.Fatal("double Close() test failed: second Close() call didn't return")
 	}
 
-	err := watcher.Watch("_test")
+	dir, err := ioutil.TempDir("", "wininotify")
+	if err != nil {
+		t.Fatalf("TempDir failed: %s", err)
+	}
+	defer os.RemoveAll(dir)
+
+	err = watcher.Watch(dir)
 	if err == nil {
 		t.Fatal("expected error on Watch() after Close(), got nil")
 	}