In just over a decade Alibaba Group Holding Ltd. turned Singles’ Day, a quirky celebration of Chinese singlehood, into the world’s biggest shopping event, drawing in hundreds of millions of people across the globe. This year’s extravaganza will be closely watched by investors keen to gauge how willing Chinese consumers are to spend in the wake of the coronavirus pandemic — and how eager U.S. merchants are to return amid Washington’s escalating drive to curb Chinese technology giants.