export tests: Improve how we check avatars.
export tests: Improve how we check avatars.

We avoid code duplication, and we iterate
over all records to see if files exist.

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