fonctional tests: use UTC timezone to avoid failures in tests sensible to DST

Some tests are sensible to Daylight saving time when they do some checking
with non fixed dates

product/ERP5Type/tests/ERP5TypeFunctionalTestCase.py

@@ -367,6 +367,7 @@ class ERP5TypeFunctionalTestCase(ERP5TypeTestCase):
   def afterSetUp(self):
     super(ERP5TypeFunctionalTestCase, self).afterSetUp()
     # create browser_id_manager
+    self.setTimeZoneToUTC()
     if not "browser_id_manager" in self.portal.objectIds():
       self.portal.manage_addProduct['Sessions'].constructBrowserIdManager()
     self.commit()

product/ERP5Type/tests/ERP5TypeTestCase.py

@@ -288,6 +288,17 @@ class ERP5TypeTestCaseMixin(ProcessingNodeTestCase, PortalTestCase):
       assert date_time is None or isinstance(date_time, DateTime)
       _pinned_date_time = date_time
 
+    def setTimeZoneToUTC(self):
+      # Make sure tests runs with UTC timezone. Some tests are checking values
+      # based on now, and this could give unexpected results:
+      # DateTime("2016/10/31") - DateTime("2016/10/30") = 1.0416666666666667 if
+      # you are running on a timezone like Europe/Paris, while it return 1.0 for
+      # UTC
+      os.environ['TZ'] = "UTC"
+      time.tzset()
+      DateTime._isDST = False
+      DateTime._localzone = DateTime._localzone0 = DateTime._localzone1 = "UTC"
+
     def unpinDateTime(self):
       self.pinDateTime(None)