Посмотрев исходный код flutter_test, я пришел к выводу:
void tap(Offset pos){
final result = HitTestResult();
WidgetsBinding.instance.hitTest(result, pos);
result.path.forEach((element) {
element.target.handleEvent(
PointerDownEvent(
localPosition: pos,
kind: PointerDeviceKind.touch),
element,
);
element.target.handleEvent(
PointerUpEvent(
localPosition: pos,
kind: PointerDeviceKind.touch),
element,
);
});
}