TAREK AMARA and ANGUS McDOWALL Two Bulgarian visitors stood in the ancient El Jem amphitheatre, one of Tunisia's top attractions, alone apart from swallows flitting under stone arches -- a sight foretelling another tourist season wrecked by COVID-19. The 3rd-century structure, so symbolic of Tunisia that it features on the 20-dinar note, usually receives about 190,000 visitors a year, but in 2020 only 45,000 came, and so far this year it has been deserted most of the time. Over the past two weeks numbers have picked up a little after the government relaxed quarantine rules for package tours to salvage…