test_auth_backends: Remove deprecated match_querystring argument.
test_auth_backends: Remove deprecated match_querystring argument.

match_querystring is irrelevant in these cases.  Fixes this warning:

/srv/zulip-py3-venv/lib/python3.7/site-packages/responses/__init__.py:340:
DeprecationWarning: Argument 'match_querystring' is deprecated. Use
'responses.matchers.query_param_matcher' or
'responses.matchers.query_string_matcher'

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

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