Add 128-bit test set

Author: mborland

test/test_downward_rounding.cpp

@@ -65,5 +65,17 @@ int main()
     test_sub<decimal_fast32_t>("1e+2", "1e-20", "99.99999");
     test_sub<decimal_fast32_t>("1e+1", "1e-20", "9.999999");
 
+    test_add<decimal128_t>("1e+2", "-1e-383", "99.99999999999999999999999999999999");
+    test_add<decimal128_t>("1e+1", "-1e-383", "9.999999999999999999999999999999999");
+
+    test_add<decimal_fast128_t>("1e+2", "-1e-383", "99.99999999999999999999999999999999");
+    test_add<decimal_fast128_t>("1e+1", "-1e-383", "9.999999999999999999999999999999999");
+
+    test_sub<decimal128_t>("1e+2", "1e-383", "99.99999999999999999999999999999999");
+    test_sub<decimal128_t>("1e+1", "1e-383", "9.999999999999999999999999999999999");
+
+    test_sub<decimal_fast128_t>("1e+2", "1e-383", "99.99999999999999999999999999999999");
+    test_sub<decimal_fast128_t>("1e+1", "1e-383", "9.999999999999999999999999999999999");
+
     return boost::report_errors();
 }