test_retention: Add type annotation for expected_result
test_retention: Add type annotation for expected_result

Otherwise mypy infers the type of `expected_result` to be incompatible
with the first argument of `fix_ordering_of_result`.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

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