Bump Zulip pin to update sliver z-order tests (#456)

This updates to the following change in Zulip:
  zulip/zulip-flutter#1480
  zulip/zulip-flutter@88e7bcc

in order to unblock this PR:
  #164818

> [!IMPORTANT]
> This repository is a testing suite that contains references to tests (in the `registry` directory) that are run with every commit to Flutter
> to verify that no breaking changes have been introduced (in the "customer_testing" shards). Your merged PR is _not_ automatically run in the
> Flutter CI tree.
>
> After merging this PR, you must send another PR to flutter/flutter updating `dev/customer_testing/tests.version` in that repo with the latest git commit SHA of the flutter/test repo.
>
> See <#162048> for details.

Author: gnprice

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 d302584e53b332d6b503b4d0a9761d4b5c419069
+fetch=git -C tests checkout 88e7bccd663f9e1f6eb9e7f4bb19fb8ffd011391
 
 setup.linux=tools/customer-testing/setup.sh