Question

What did God want to test in human beings by sending the plate of gold

Answer 1

God want to check the honesty and humanity by sending different types of gifts to humans . By this god concluded that  mostly persons are greedy in his world so later he stop sending his blessings . by the world of greedy people we made this world very bad . we are exploiting it day by day . God want to let us learn a lesson now 

Answer 2

God wants to test in human beings by sending the plate of gold because he wants to test the people honesty, humanity and simplicity towards anything in this world.People often feel greedy when he sees something very useful to him or either very costly. Mostly greedy people tell lie.

we can see this humanity in one of your known stories i.e., "the woodcutter and axe" where the honest woodcutter tells the truth to the water-fairy in another hand the other woodcutter tells lie to the water-fairy.

so, we should always be honest.