Mock jed for test determinism

What does this MR do and why?

This MR adds a __mocks__/jed/index.js so that we won't actually translate strings in unit tests. This would allow us to remove the test file externalization requirement for JavaScript unit tests (see current relevant docs).

From the commit message:

• #390934 (comment 1494028934)
• It'a possible for some test environments to run in a specific locale. This change skips pulling translations in unit tests to improve test determinism across environments.

How to set up and validate locally

1. Run bin/rake gettext:compile
2. Add an import to any of the locale/*/app in test_setup.js

   diff --git a/spec/frontend/test_setup.js b/spec/frontend/test_setup.js
   index d3d3e5c8c720..85f0711c135b 100644
   --- a/spec/frontend/test_setup.js
   +++ b/spec/frontend/test_setup.js
   @@ -3,6 +3,7 @@
    import { setImmediate } from 'timers';
    import Dexie from 'dexie';
    import { IDBKeyRange, IDBFactory } from 'fake-indexeddb';
   +import '~/locale/es/app';
    import 'helpers/shared_test_setup';
    
    const indexedDB = new IDBFactory();
   

3. Run jest for a test that includes a hardcoded non-translated string in the assertion. Example yarn run jest ee/spec/frontend/pages/admin/application_settings/ci_cd/ci_template_dropdown_spec.js.

Thanks

Thanks @afontaine for your help on this! https://youtu.be/0-6gspPSFkA

MR acceptance checklist

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