tests: fix C++ conformance

Reported by Thomas Jahns.
http://lists.gnu.org/archive/html/bug-bison/2015-01/msg00059.html

* tests/c++.at (Exception safety): Add missing include.
Don't use const_iterator for erase.

tests/c++.at

@@ -719,6 +719,7 @@ $1
   #include <cstdlib> // size_t and getenv.
   #include <iostream>
   #include <set>
+  #include <string>
 
   bool debug = false;
 
@@ -751,7 +752,7 @@ $1
     ~Object ()
     {
       log (this, "Object::~Object");
-      objects::const_iterator i = instances.find (this);
+      objects::iterator i = instances.find (this);
       // Make sure this object is alive.
       assert (i != instances.end ());
       Object::instances.erase (i);