asdf: make test-op do something

Hi,

I think there should be

(defmethod operation-done-p ((o test-op) c)
   nil)

in asdf -- right now, it's handled by the default operation-done-p 
method that returns t for (operation, component) pairs that have 
neither input- nor output-files.  This means that (asdf:operate 
'asdf:test-op <something>) does nothing by default.

I could commit such a change if there are no objections.

Cheers,

Rudi