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

[The Saucepan Man]

Remember, she also says "I will diminish, and go into the West, and remain Galadriel." Her original reason for leaving Valinor was her wish to rule her own kingdom. She sees the return to Valinor as a diminishing - ie not an entirely positive thing

Good point, Lalaith. This heightens the significance of the "test" presented to her by the Ring. Not only was she giving up the power it promised her to rule over others as a Dark Queen, but she was also giving up her current power and status as ruler of a realm in ME. Although she would no doubt have been accorded great status on her return to Valinor, it was unlikely to be quite the same as being Lady of the "heart of Elvendom on earth".