BEIJING—Hong Kong authorities on Saturday asked the entire population of more than 7.4 million people to voluntarily test themselves for COVID-19 at home for three days in a row starting next week. The announcement by Chief Executive Carrie Lam came as the southern Chinese city is struggling to contain its worst outbreak with authorities sending mixed signals about testing and lockdowns. Lam said a “compulsory, universal test” of the whole population is still essential but did not say when that might happen. Authorities shelved the idea after a previous announcement caused panic buying. The prospect of further school closures and other disruptions has the government caught between calls for loosening restrictions and Beijing’s demand for an extreme “zero-COVID” approach mandating lockdowns and mass testing. Hong Kong, a semi-autonomous territory, on Friday lifted a ban on residents returning aboard flights from nine countries where COVID-19 cases have surged, including Britain and …