Hong Kong authorities have created a Covid-19 test booking platform online for travellers bound for mainland China to ease overcrowding at the Shenzhen border, but have done little so far to plug a loophole that allows those being turned away after testing positive to simply return home without any immediate restrictions on their movements or follow-up isolation orders. Only those with proof of a reservation will be allowed to undergo testing from Friday at the port, one of two land crossings in operation between the mainland and Hong Kong. Users can log on to book time slots for tests over…