tests: dnd2: Fix image DnD

image_drag_data_get() checks the target type in a way that results in gtk_selection_data_get_targets() being called before the data has been set, which, in turn, results in an assertion.

The other problem is image_drag_data_received() doing the same, but failing due to the selection data type not satisfying the condition (gtk_selection_data_set_pixbuf() passes 8, but 32 is expected).

Both of those can be fixed by checking the atom received from gtk_selection_data_get_target().