Use std::advance in test::next, and use it instead of boost::next

Mainly to avoid warnings from boost::next

test/helpers/helpers.hpp

@@ -6,6 +6,8 @@
 #if !defined(BOOST_UNORDERED_TEST_HELPERS_HEADER)
 #define BOOST_UNORDERED_TEST_HELPERS_HEADER
 
+#include <iterator>
+
 namespace test {
   template <class Container> struct get_key_impl
   {
@@ -36,16 +38,16 @@ namespace test {
   // test::next
   //
   // Increments an iterator by 1 or a given value.
-  // Like boost::next, but simpler and slower.
+  // Like boost::next, but simpler.
+  // Mainly because boost::next uses an MPL file
+  // which causes warnings.
 
   template <typename Iterator> Iterator next(Iterator it) { return ++it; }
 
   template <typename Iterator, typename IntType>
   Iterator next(Iterator it, IntType x)
   {
-    for (; x > 0; --x) {
-      ++it;
-    }
+    std::advance(it, x);
     return it;
   }
 }