The tango is based on suspicion, sex and insincerity. It is not a dance for virgins. It is for the wounded and the wary. The opening shots of Carlos Saura's "Tango,'' after a slow pan across Buenos Aires, are of a man who has given his life to the dance and has a bad leg and a walking stick as his reward. This is the weary, graceful Mario (Miguel Angel Sola), who is preparing a new show based on the tango.