This should work. I'm sorry, I conformed it to my own style (lowercase and different indentation).

    with weeks as (
        select week
        from generate_series(
            '2017-01-01'::date,
            now()::date,
            '1 week'::interval
        ) week
    )
    select weeks.week, count(1)
    from weeks, test_results
    where
        test_results.date_created > weeks.week and
        test_results.date_created <= (weeks.week - '1 week'::interval)
    group by week
    ;