tests: Improve testing helper event schema for `update_message`.
tests: Improve testing helper event schema for `update_message`.

Further clarifies the fields returned by `update_message` event
for the type of change (content, topic and/or stream).

Follow-up task from #20587.

eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,andersk/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,zulip/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip