Generate Deeply Nested Frontend Fixtures

The original default pattern, `frontend/fixtures/*.rb`, only searches in
the `fixtures` directory for rspec files. `**/*.rb` is needed to find
rspec files in nested directories, such as those in the
`ee/spec/frontend/fixtures/analytics` folder.

lib/tasks/frontend.rake

@@ -5,7 +5,7 @@ unless Rails.env.production?
       directories = %w[spec]
       directories << 'ee/spec' if Gitlab.ee?
       directory_glob = "{#{directories.join(',')}}"
-      args.with_defaults(pattern: "#{directory_glob}/frontend/fixtures/*.rb")
+      args.with_defaults(pattern: "#{directory_glob}/frontend/fixtures/**/*.rb")
       ENV['NO_KNAPSACK'] = 'true'
       t.pattern = args[:pattern]
       t.rspec_opts = '--format documentation'