Galadriel knew about the Test (the tempation of the One Ring)... But how??

[Legolas]

. And Ilúvatar never interfered in the doings of Arda after creation except for the sundering of Aman.

Wrong. It was Eru who sent Gandalf back to Middle-earth. It was Eru who gave Tuor immortality. It was Eru who gave Luthien a choice between immortality and mortality. It was Eru who placed Beren and Luthien back in the world together. It was Eru who gave the half-elven of Earendil and Elwing's house the choice between mortality and immortality. It was Eru who destroyed Numenor.