test_subs: Use HostRequestMock for request object.
test_subs: Use HostRequestMock for request object.

I incorrectly removed this when simplifying
dbddbee5a115b9352862cb13d4c66820865c30b6; while that commit did not
require the hunk re-added here, the later commit
3be622ffa7693d1b4540aee2df287c41529fb82f added a call that did require it.

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