Adapt llimageworker_test for updated virtual method API.

This was a broken test that got all the way to viewer release and the main
branch.

(cherry picked from commit a33a9d29380e6c1a0a9cc539be309d47adef4acf)

indra/llimage/tests/llimageworker_test.cpp

@@ -98,7 +98,7 @@ namespace tut
 				done = res;
 				*done = false;
 			}
-			virtual void completed(bool success, LLImageRaw* raw, LLImageRaw* aux)
+			virtual void completed(bool success, LLImageRaw* raw, LLImageRaw* aux, U32)
 			{
 				*done = true;
 			}