It sounds like a setup for a parlor game: If you could get 19 of the best female voices in New Orleans together for one song, which one would it be? That was answered during French Quarter Festival this year, when all 19 official members of the New Orleans Nightingales were onstage together for the first, and so far only time. And the song they chose was “Laissez Faire,” a tune that’s been a staple of the