US President Donald Trump and his Democratic challenger, Joe Biden, will face off for their second and final debate on Thursday night, a crucial test for Trump, who is trailing in polls as the nation continues to reel from a devastating pandemic his administration has failed to control.Covid-19 is one of six subjects on the agenda for the debate at Belmont University in Nashville, Tennessee, along with American families, race, climate change, national security and leadership.Organisers are…