test_subs: Enforce invite_only argument to be named.
test_subs: Enforce invite_only argument to be named.

This commit enforces invite_only argument to be named
in _test_user_settings_for_creating_streams. This will
help in improving readability especially when we will
add is_web_public argument in further commits.

eeshangarg/zulip,kou/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip