message_edit: Test is_topic_editable separately.
message_edit: Test is_topic_editable separately.

We now do not test is_topic_editable function in
get_editability test for better readability and
this will be helpful when we will need to test
more cases as part of #21739.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip