Several officials of Japanese ad giant Dentsu Inc. have admitted to collusion over bid rigging for contracts related to test events for the Tokyo Olympic and Paralympic Games, a source familiar with the matter said Friday. The officials admitted to the wrongdoing during voluntary questioning with prosecutors, while maintaining they were not aware of any illegality at first when the bid rigging took place, according to the source. A former operations executive of the event’s organizers is suspected of playing a lead role in the rigging, along with several Dentsu staff, some of whom were seconde…