api: Handle restart events in apply_events.

Event of type restart could not be handled properly, because of
its special behavior. For handling this event in most natural way
we recursively call `do_events_register` when restart event is
recieved, based on custom error created for this event.

Testing: Second call to get_user_events due to recursive calling
of do_event_register, is expected to not contain the restart event.
So new test added in test_event_system.py are based on above behavior
of get_user_events.

Fixes: #15541.

zerver/lib/test_helpers.py

@@ -51,6 +51,7 @@ from zerver.models import (
     Subscription,
     UserMessage,
     UserProfile,
+    get_client,
     get_realm,
     get_stream,
 )
@@ -327,10 +328,13 @@ class HostRequestMock:
         post_data: Dict[str, Any] = {},
         user_profile: Optional[UserProfile] = None,
         host: str = settings.EXTERNAL_HOST,
+        client_name: Optional[str] = None,
     ) -> None:
         self.host = host
         self.GET: Dict[str, Any] = {}
         self.method = ""
+        if client_name is not None:
+            self.client = get_client(client_name)
 
         # Convert any integer parameters passed into strings, even
         # though of course the HTTP API would do so.  Ideally, we'd