Yes, ImageList creates a copy of the bitmap. But your test code runs afoul of the famous lazy initialization pattern that's so common in the .NET framework. What matters is when it creates the copy. Which is does only when it has to. Make a small change in your code to hurry that up:
il.Images.Add(test);
var dummy = il.Handle; // <== NOTE: added
test.Dispose(); // no problem
And you'll see that disposing is no longer a problem.
Not sure how to give proper advice here, the code is too synthetic. This in general works well enough, ImageList makes the copy when its consumers start using its bitmaps, Treeview or ListView. In general, avoid using ImageList as a collection object, it wasn't made to do that job. Separate the view from the model and you'll stay out of trouble.