Add zulip.test (#368)

Zulip is an open-source team chat app, with a new Flutter-based
mobile client now in beta. This is that client's test suite.

I believe these will be the only tests currently in this registry
for an app, rather than a library. That should naturally give them
a different mix of use cases and types of tests.

Concretely, we've seen a handful of breaking changes over the past
year that weren't caught by any of Flutter's existing test suites.
These were in text hit-testing:
  zulip/zulip-flutter@ba7a2bf
  flutter/flutter#140621

and Material menu behavior:
  zulip/zulip-flutter@38ed6c8
  flutter/flutter#130536

and SlottedContainerRenderObjectMixin gaining a type parameter:
  zulip/zulip-flutter@2f0f469
  flutter/flutter#126108

I'm not complaining, to be clear, and none of these were
particularly onerous for us to adapt to. By registering these
tests, I'm hoping to provide feedback on future such breakages
at a point where it's actionable.

Omitted here are several tests that re-generate generated files
and check they match what's in the tree. Those are pretty slow,
and I think they're pretty insensitive to changes in the Flutter
tree anyway; rather they depend on pigeon, json_serializable,
build_runner, and drift_dev.

registry/zulip.test

@@ -0,0 +1,11 @@
+contact=greg@zulip.com
+contact=cbobbe@zulip.com
+
+fetch=git clone https://github.com/zulip/zulip-flutter.git tests
+fetch=git -C tests checkout 9ea05846858fe36a9820f01917eebce466cb8456
+
+update=.
+update=packages/zulip_plugin
+
+test.posix=flutter analyze --no-fatal-infos
+test.posix=flutter test