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

package.json

@@ -6,7 +6,7 @@
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "tsc": "yarn workspaces run tsc",
-    "gentest": "node --loader=babel-register-esm gentest/gentest-driver.ts"
+    "gentest": "yarn workspace gentest run gentest"
   },
   "workspaces": [
     "javascript",