tests: Improve tests for unsubscribing multiple users.
tests: Improve tests for unsubscribing multiple users.

Note that the tests now reflect that we have O(N)
behavior for multiple users.

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