zendesk tests: Fix anti-patterns.
zendesk tests: Fix anti-patterns.

We had optional parameters for expected_topic and
expected_message, which are trivial to eliminate,
since the integration is really simple.

And we were doing strange things trying to reset
class variables at the end of tests.  Now we just
set them explicitly in the tests.

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