provide test for initializer list construction

dune/common/test/diagonalmatrixtest.cc

@@ -67,6 +67,14 @@ void test_interface()
   checkMatrixInterface< DiagonalMatrix, Traits >( A );
 }
 
+void test_initialisation()
+{
+  Dune::DiagonalMatrix<int, 2> const b = { 1, 2 };
+
+  assert(b.diagonal(0) == 1);
+  assert(b.diagonal(1) == 2);
+}
+
 int main()
 {
   try {