Use a timezone that does not use DST in tests

I was getting a failure in the scheduled publishing tests, as in 10 days
time exactly the America/Chicago timezone goes through a DST transition,
which renders a time submitted in a form to be invalid.

Debugging this was fun.

wagtail/tests/settings.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 import os
+
 import django
 
 WAGTAIL_ROOT = os.path.dirname(os.path.dirname(__file__))
@@ -8,6 +9,8 @@ STATIC_ROOT = os.path.join(WAGTAIL_ROOT, 'tests', 'test-static')
 MEDIA_ROOT = os.path.join(WAGTAIL_ROOT, 'tests', 'test-media')
 MEDIA_URL = '/media/'
 
+TIME_ZONE = 'Asia/Tokyo'
+
 DATABASES = {
     'default': {
         'ENGINE': os.environ.get('DATABASE_ENGINE', 'django.db.backends.sqlite3'),