More than 3,000 medical professionals in Hong Kong have signed up to help operate the citywide coronavirus testing scheme aimed at cutting off invisible transmission chains, as top officials on Sunday drummed up support for the programme.The authorities said the scheme, set to launch in September, would identify more asymptomatic carriers, accounting for more than 20 per cent of confirmed cases so far, if more people took part.They called on the public to put aside bias and conspiracy theories…