Add an expression to the CORE_Expr test that should be similiar to what we observe in Envelope_3

Number_types/test/Number_types/CORE_Expr.cpp

@@ -137,6 +137,10 @@ int main() {
 
     CGAL::test_real_embeddable<NT>();
 
+    CORE::BigRat br(1,3);
+
+    CORE::Expr exp = 2 * br;
+
   return 0;
 }