Los Angeles is not known for long theatre runs. But “Wicked” plans to stay a while. “Two years may sound like a tall order for a city in which theater people must ever grapple with the overweening presence of the film and television industry. What’s more, because few open-ended commercial engagements have done well here in recent decades, L.A. is sometimes touted as a tough town for sit-downs. Yet insiders predict success.”