By Daniela Altimari and Mary Ellen McIntireSaturday’s Democratic primary in Hawaii’s 1st District will provide another test of the power of incumbency against the thirst for change gripping the party’s grassroots. Already, six House Democrats have lost primary challenges in deep-blue districts this year. Hawaii state Sen. Jarrett Keohokalole is aiming to make Rep. Ed Case the seventh. “The solutions we’ve been getting from Ed Case are not good enough,’’ Keohokalole said in a campaign spot. The winner of the Democratic primary will be heavily favored to win the deep-blue seat in November.