LONDON — Labour MPs know Keir Starmer is unpopular. On Thursday, actual voters will get a chance to show just how deep that discontent runs. A by-election in the Greater Manchester constituency of Gorton and Denton is being billed as a major test for the embattled British prime minister. Once considered a safe seat for his ruling Labour Party, the constituency is now being challenged by populists on the right and left, and the race — triggered by an existing Labour MP stepping down on health