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).
Indeed, simply setting get_default_timezone() like this doesn't work for pytz timezones that have DST.