Workers who allege their supervisors feel free to use racial slurs have been given the go-ahead to jointly pursue a lawsuit against their employer. In a written opinion, U.S. District Judge Matthew F. Kennelly did not rule on the merits of the claim that black employees at Comcast Corp. Inc.’s facility on the city’s South Side are subjected to a hostile work environment. But Kennelly held that the 12 employees who sued Comcast presented enough evidence to warrant granting their motion for class …