A trial is underway in the federal government’s lawsuit aiming to kill a partnership between American Airlines and JetBlue Airways in the Northeast. The government says the deal is, in effect, a merger that will cost consumers $700 million a year in higher fares. But the airlines say the pact — which has been in place for 18 months — is already letting them open new routes that are good for travelers. They say the deal boosts competition by helping American and JetBlue compete with Delta and United, especially in the New York area.