katiestar, the McCanns owe you nothing. No explanation. No test. No apology.
If they have done anything criminal then it is a criminal matter, and the police will question/ charge them as they deem fit.
The opinion of the public is neither here nor there. There are no end of people willing to concoct conspiracy theories on the internet, on everything from space exploration to 9/11.
This is just amateur speculation, and carries no legal weight whatsoever.
Anyway, given how far the conspiracy theorists are willing to take their arguments, if any lie detector test was to 'clear' the McCanns of whatever is is that 'lots of people' think they have done, they'd just say it was a whitwash, cover up, another McCann PR coup etc etc.