Update zulip.test to latest SHA. (#436)

Part of flutter/flutter#160257.

Zulip recently removed usages of `flutter_gen`, which are deprecated pending removal.

Author: matanlurey

registry/zulip.test

@@ -2,7 +2,7 @@ [email protected]
 [email protected]
 
 fetch=git clone https://github.com/zulip/zulip-flutter.git tests
-fetch=git -C tests checkout 753a9b4b35d652632f61c18a7ba7b7ef83053a3e
+fetch=git -C tests checkout 28b3536bc81b6665a6014ce72f40c51f83b0c5c4
 
 update=.
 update=packages/zulip_plugin