Unit testing in Wicket

[T P D]

Please help me to understand how testing works in Wicket. (Not how to 
test in the Wicket Framework, but how to test the Wicket Framework 
classes themselves.)

I see that DropDownChoice is tested in a Page, in
org.apache.wicket.markup.html.form.DropDownChoiceTestPage
But I don't see a unit test for DropDownChoice, and it's not immediately 
 obvious to me how DropDownChoiceTestPage exercises a DropDownChoice in 
isolation. Is there a unit test for DropDownChoice that I'm not seeing?

Thanks,
Tom