[1.6.x] Factorized requires_tz_support decorator in test utils

Thanks Aymeric Augustin for the suggestion. Refs #21165.
Backport of c1c44b2506 from master.

tests/generic_views/test_dates.py

@@ -1,27 +1,14 @@
 from __future__ import absolute_import
 
-import time
 import datetime
 
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase, skipUnlessDBFeature
-from django.test.utils import override_settings
+from django.test.utils import override_settings, requires_tz_support
 from django.utils import timezone
-from django.utils.unittest import skipUnless
 
 from .models import Book, BookSigning
 
-TZ_SUPPORT = hasattr(time, 'tzset')
-
-# On OSes that don't provide tzset (Windows), we can't set the timezone
-# in which the program runs. As a consequence, we must skip tests that
-# don't enforce a specific timezone (with timezone.override or equivalent),
-# or attempt to interpret naive datetimes in the default timezone.
-
-requires_tz_support = skipUnless(TZ_SUPPORT,
-        "This test relies on the ability to run a program in an arbitrary "
-        "time zone, but your operating system isn't able to do that.")
-
 
 def _make_books(n, base_date):
     for i in range(n):