[matrix test]

avoid "variable unused" warning
in this case using DUNE_UNUSED is okay, since we just want to test the
existence of the copy constructor...

This is FS#1165 patch 2.
(thanks to Andreas Lauser for the patch)

[[Imported from SVN: r1717]]

dune/istl/test/matrixtest.cc

@@ -197,7 +197,7 @@ void testMatrix(MatrixType& matrix, X& x, Y& y)
   matrix = 0;
 
   // The copy constructor
-  MatrixType thirdMatrix(matrix);
+  DUNE_UNUSED MatrixType thirdMatrix(matrix);
 
   // ///////////////////////////////////////////////////////
   //   Test component-wise operations