tests/gdatetime: fix new_from_unix test

When creating a struct tm for "1990-01-01T00:00:00" to pass to
mktime(), we have to set tm_isdst to -1; leaving it set to 0 will
result in the wrong time being generated when run in a timezone where
January 1 would normally be tm_isdst==1 (ie, in southern hemisphere
DST-observing countries, like Australia).

https://bugzilla.gnome.org/show_bug.cgi?id=670254

glib/tests/gdatetime.c

@@ -114,6 +114,7 @@ test_GDateTime_new_from_unix (void)
   tm.tm_hour = 0;
   tm.tm_min = 0;
   tm.tm_sec = 0;
+  tm.tm_isdst = -1;
   t = mktime (&tm);
 
   dt = g_date_time_new_from_unix_local (t);