Böyle dener misiniz?
SELECT SUM(c.test_sonuc) AS toplam, cci.user_id
FROM test_cevap AS c
	INNER JOIN test_cevapci AS cci ON (c.cevap_id = cci.cevap_id)
WHERE cci.user_id = 1234
ORDER BY c.cevap_id DESC
LIMIT 10