GMO food is actually tested in the same way all other food is tested for safety.
And "GMO" doesn't actually mean anything. Using standard hybridization techniques that were developed 5,000 years ago, you can "genetically modify" how a plant grows, and we continue to do it to this very day.
Anybody that understands basic biology, or works on a farm, knows the basic techniques for getting a plant to grow differently to emphasize a particular feature, like seedless grapes, or yellow tomatoes.
The change that people are worried about with "GMOs" is that these genes are being spliced directly in a lab, and inserted into the plant, rather than waiting for the mutation to occur naturally, and then encouraging that mutation.
Can science go too far? Yeah. Maybe. But it hasn't. The food they make is still food by every measure we have managed to come up with.
Will this always be true? Who knows?
THAT is why you can't say any food is "safe". You just don't know what we may uncover in the future, regardless of whether there is gene splicing going on or not.