tests: Create export files for specific user.
tests: Create export files for specific user.

We no longer hackily look for the first message ever
sent within the realm.

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