WASHINGTON — To hear President Donald Trump tell it, he was made for a moment like this: A high-stakes face-off. A ticking clock. A cagey adversary. The man who calls himself a supreme dealmaker will have the opportunity this week to put himself to the test. The question is whether he can defuse a trade war with China that is shaking financial markets and threatening the global economy — and perhaps achieve something approximating a breakthrough. Trump is to meet with his Chinese counterpart, Xi Jinping, during the Group of 20 summit in Buenos Aires, Argentina, on Friday and Saturday.