QDateTime auto test: stabilize currentDateTimeUtc2 test

... by trying more often to get dates close enough to each other.

Change-Id: I370f7cd61bbb84fbb77ea96ff9fd82c1a6f1f76a
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>

tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp

@@ -1116,7 +1116,7 @@ void tst_QDateTime::currentDateTimeUtc2()
     qint64 msec;
 
     // check that we got all down to the same milliseconds
-    int i = 2;
+    int i = 20;
     bool ok = false;
     do {
         local = QDateTime::currentDateTime();