More than half of Shanghai’s 25 million residents will undergo Covid-19 testing this weekend, raising the prospect of extended periods of home confinement for any who test positive, a rude awakening that comes just one week after the city celebrated its emergence from a punishing monthslong lockdown. On Thursday, Shanghai authorities ordered residents in at least seven of the city’s districts, with a combined population of 14 million, to undergo mass Covid-19 testing starting Saturday, according to state media reports. Authorities didn’t give details on how long the sampling period will last.