tests: fix -Wdeprecated warning

With recent compilers:

    input.yy:49:5: error: definition of implicit copy assignment
                          operator for 'Object' is deprecated because
                          it has a user-declared destructor
                          [-Werror,-Wdeprecated]
        ~Object ()
        ^
    input.yy:130:35: note: in implicit copy assignment operator for
                           'Object' first required here
        { yylhs.value.as< Object > () = yystack_[0].value.as< Object > (); }

* tests/c++.at (Object): Add missing assignment operator.

tests/c++.at

@@ -770,6 +770,12 @@ $1
       Object::instances.erase (i);
     }
 
+    Object& operator= (const Object& that)
+    {
+      val = that.val;
+      return *this;
+    }
+
     Object& operator= (char v)
     {
       val = v;