test: Improve tests for bulk-adding subscribers.
test: Improve tests for bulk-adding subscribers.

This is a more thorough test of adding multiple
streams for multiple users, including streams
that users have already subscribed to.

The extra queries here are due to the fact
that we call `principal_to_user_profile` in
a loop in the view.  So that's an example
of O(N) overhead.  We may be able to bulk-fetch
these users eventually.

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