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/gentest.js

@@ -53,12 +53,12 @@ function printTest(e, ext, LTRContainer, RTLContainer, genericContainer) {
     ' *',
     ' * This source code is licensed under the MIT license found in the',
     ' * LICENSE file in the root directory of this source tree.',
-    ' */',
-    ext === 'cpp' ? '\n// clang-format off' : '',
-    '// @' +
-      'generated by gentest/gentest-driver.ts from gentest/fixtures/' +
+    ext === 'cpp' ? ' *\n * clang-format off' : ' *',
+    ` * MAGIC_PLACEHOLDER`,
+    ' * generated by gentest/gentest-driver.ts from gentest/fixtures/' +
       document.title +
       '.html',
+    ' */',
     '',
   ]);
   e.emitPrologue();