test_auth_backends: Verify the avatar contents.
test_auth_backends: Verify the avatar contents.

Consuming response.streaming_content fixes this warning from
TestLDAP.test_login_success_when_user_does_not_exist_with_valid_subdomain:
“ResourceWarning: unclosed file <_io.FileIO
name='/srv/zulip/var/…/3cc4b5a15b6f4f06b3f9a6ecb179b08702329716.png'
mode='rb' closefd=True>”.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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