Fixed #23893: Added tzinfo to constant datetime in unit test

Added tzinfo to y2k constant (01/01/2000) in
custom_lookups.tests.DateTimeLookupTests.test_datetime_output_field
to fix warning message regarding naive datetimes.

Using default timezone from config (since pytz may not be available).
You can use `django.utils.timezone.make_aware` here: https://docs.djangoproject.com/en/dev/ref/utils/#django.utils.timezone.make_aware