Sign generated tests (#1503)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1503

This diff makes it so that our driver will sign all of the generated files to help ensure that they are not edited by hand. Next I will add CI to actually verify the signature

Reviewed By: NickGerleman

Differential Revision: D51966201

fbshipit-source-id: f7e3f4fde1c98832212a448b2dcc8e21be0560c4

gentest/signedsource.d.ts

@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+declare module 'signedsource';