Add support for UTC Date for testing purpose

The tests may depend on the timezone of the browser so we need a way to write
tests as UTC only.

issue9577
review298511002

src/pyson.js

@@ -745,10 +745,10 @@
     Sao.PYSON.Date.eval_ = function(value, context) {
         var date = value.start;
         if (date && date.isDateTime) {
-            date = Sao.Date(date.year(), date.month(), date.date());
+            date = Sao.Date(date.year(), date.month(), date.date(), true);
         }
         if (!date || !date.isDate) {
-            date = Sao.Date();
+            date = Sao.Date(undefined, undefined, undefined, true);
         }
         if (value.y) date.year(value.y);
         if (value.M) date.month(value.M - 1);

src/sao.js

@@ -168,7 +168,7 @@ var Sao = {};
 
     Sao.Decimal = Number;
 
-    Sao.Date = function(year, month, day) {
+    Sao.Date = function(year, month, day, utc) {
         var date;
         if (month === undefined) {
             date = moment(year);
@@ -177,6 +177,9 @@ var Sao = {};
         else {
             date = moment();
         }
+        if (utc) {
+            date.utc();
+        }
         date.year(year);
         date.month(month);
         date.date(day);