Fix wrong std::cerr at end of output.

It should be std::endl.
Courtesy of GCC 5-svn, it refuses to compile these.

dune/common/test/iteratortest.hh

@@ -182,7 +182,7 @@ int testRandomAccessIterator(Iter begin, Iter end, Opt opt){
     if(test != rand)
     {
       std::cerr << "i+=n should have the same result as applying the"
-                << "increment ooperator n times!"<< std::cerr;
+                << "increment ooperator n times!"<< std::endl;
       ret++;
     }
 
@@ -201,7 +201,7 @@ int testRandomAccessIterator(Iter begin, Iter end, Opt opt){
     if(test != rand)
     {
       std::cerr << "i+=n should have the same result as applying the"
-                << "increment ooperator n times!"<< std::cerr;
+                << "increment ooperator n times!"<< std::endl;
       ret++;
     }
   }