GFI trips when light fixture plugged in
It's an open-and-shut case of a ground fault

All your testing (two GFCIs, isolating appliance from earth etc.) confirms the appliance has a ground fault. 

Your tester doesn't detect it because your tester is not a megaohmmeter.  The particular failure mode you have here is insulation breakdown; it's fine at the 3 volt meter test voltage, not so good at 120/230V line voltage and will certainly fail at the megaohmmeter's 200V/500V test voltage.  

Just dismantle the light and clean everything and look for any obvious problems.