Re-land "Update Zulip tests and re-enable (#441)" (#446)

This re-lands 8cf7a67 / #441.

Difference from previous version: This version has the new setup step do its work only where `sudo` is available (in GitHub Actions), and successfully do nothing where it's unavailable (in LUCI).  The step isn't needed in LUCI.

The logic lives in a setup script added today to the Zulip tree:
  zulip/zulip-flutter#1300

registry/zulip.test.disabled → registry/zulip.test

@@ -2,7 +2,9 @@ [email protected]
 [email protected]
 
 fetch=git clone https://github.com/zulip/zulip-flutter.git tests
-fetch=git -C tests checkout 28b3536bc81b6665a6014ce72f40c51f83b0c5c4
+fetch=git -C tests checkout ae7939afabb4c6a32fa5ffd0be03928467c8cd64
+
+setup.linux=tools/customer-testing/setup.sh
 
 update=.
 update=packages/zulip_plugin