A year and a half since the United States’ revamped trade agreement with Canada and Mexico went into effect, changes in the North American auto industry have created some of the greatest tests of the new deal. That was on display this week as thousands of workers at a General Motors Co. plant in Silao, Mexico, voted to join an independent union that might help them secure better wages and working conditions. The vote came after decades spent organized with a traditional Mexican labor union with a reputation for corruption. The election was closely watched by U.S. officials, who view it as one …