By Dave Graham MEXICO CITY (Reuters) -Mexicans started casting votes in six state elections on Sunday that are likely to boost President Andres Manuel Lopez Obrador’s party, in one of the last major tests of his electoral clout before the political focus shifts to the race to succeed him. Lopez Obrador’s leftist National Regeneration Movement (MORENA) and its allies are tipped to win most of the governorships up for grabs, tightening the party’s grip on the political landscape against a fractured opposition. Jointly accounting for about one-tenth of Mexico’s 126 million population, Tamaulipas …