webhook tests: Require args for check_webhook.
webhook tests: Require args for check_webhook.

We tighten up the mypy types here.  And then
once we know that expected_message and expected_topic
are never None, we don't have call the do_test_message
and do_test_topic helpers any more, so we eliminate
them, too.

Finally, we don't return a message, since no tests
use the message currently.

showell/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,zulip/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,rht/zulip,andersk/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip