I'm not particularly sure that this is what you want. You want to update or insert into the test_user table depending on the records in test_answer, right? If that is the case, this might be the answer:
INSERT INTO test_user (user_id, time_answered)
(
SELECT user_id, SUM(answered)
FROM test_answer
WHERE answered = 1 AND user_id NOT IN (SELECT user_id FROM test_user)
GROUP BY user_id
);
UPDATE test_user
SET time_answered = SUM(answered)
FROM test_answer
WHERE test_answer.user_id = test_user.user_id;
