tests: clarify some magic constant

[Akim Demaille]

commit e4fd17bffa3dccaa1d0d9b6947f20d8aa5c43893
Author: Akim Demaille <akim.demai...@gmail.com>
Date:   Tue Nov 13 07:44:52 2018 +0100

    tests: clarify some magic constant
    
    * tests/c++.at (C++ Variant-based Symbols Unit Tests): here.

diff --git a/tests/c++.at b/tests/c++.at
index df29607a..81c61863 100644
--- a/tests/c++.at
+++ b/tests/c++.at
@@ -168,13 +168,15 @@ int main()
   {
     parser::stack_type st;
     const int mucho = 1700;
+    const int int_reduction_state = 1; // Read list.output to find it.
     for (int i = 0; i < mucho; ++i)
       {
 #if defined __cplusplus && 201103L <= __cplusplus
-        st.push(parser::stack_symbol_type{1, parser::make_INT (i)});
+        st.push(parser::stack_symbol_type{int_reduction_state,
+                                          parser::make_INT (i)});
 #else
         parser::symbol_type s = parser::make_INT (i);
-        parser::stack_symbol_type ss (1, s);
+        parser::stack_symbol_type ss (int_reduction_state, s);
         st.push (ss);
 #endif
       }