its pretty easy and you have a number of choices.
1) break into the feed on the abs control relay- that will result in a self test each time that you turn it back on. This is accessible where the abs loom joins the main one.
2) Alternatively you could override the yaw sensor so that the car thinks its permanently cornering hard- accessible at the same connector.
In both cases a loop to one of the spare switched should do it.
But now the main point, I'm with Clee, don't do it.
The GTA has a weight distribution that means the front wheels will lock early- depriving you of steerage and resulting in ploughing straight on. I find it difficult (except in snow) to think of a scenario where turning the ABS off will make things better. Early Quattro's allowed you to do it because of snow.
I have a mate who thought he could do better than his M3's traction control system. Played with it, left it off accidentally, put it in a ditch (rather than the 911 coming the other way).
He had it repaired (only 4k) but has sold it on as he doesn't trust himself any more.
Sorry for the cautionary tales, but.....there are few enough GTA's left in the UK, and we'd also be sorry to see you hurt.