Use lowercase @generated in tests

Summary:
The presence of lowercase "generated" anywhere in a file means Phabricator and Meta's fork of VSCode will treat the file as generated. Change generated tests to use the exact string.

Changelog:
[Internal]

Reviewed By: yungsters

Differential Revision: D41312171

fbshipit-source-id: 2bc8ef450d8377ffbacf443043d115a418db4a2e

gentest/gentest.js

@@ -52,7 +52,7 @@ function printTest(e, ext, LTRContainer, RTLContainer, genericContainer) {
     ' * LICENSE file in the root directory of this source tree.',
     ' */',
     ext === 'cpp' ? '\n// clang-format off' : '',
-    '// @Generated by gentest/gentest.rb from gentest/fixtures/' + document.title + '.html',
+    '// @' + 'generated by gentest/gentest.rb from gentest/fixtures/' + document.title + '.html',
     '',
   ]);
   e.emitPrologue();