fix local backend test

The local backend can't define a StateOut path if we want to test
writing multiple named state files. Use a default local backend.

backend/local/backend_test.go

@@ -21,7 +21,8 @@ func TestLocal_impl(t *testing.T) {
 }
 
 func TestLocal_backend(t *testing.T) {
-	b := TestLocal(t)
+	defer testTmpDir(t)()
+	b := &Local{}
 	backend.TestBackend(t, b, b)
 }