Fixed reverse test execution for migration manager tests

Thanks to Tim Graham for reporting the issue; refs #23822
Fixed refs #23987 test on Oracle.
Refs #23947 -- Improved migrations tests table cleanup.

Copied technique from schema tests.
Refs #23884 -- Silenced a deprecation warning in sitemaps tests.
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.
Merge pull request #3737 from mhall1/ticket_23893

Fixed #23893: Added tzinfo to constant datetime in unit test
Fixed #23995 -- Removed unnecessary calls to self.client.logout() in tests.
Fixed #23983 -- Fixed a crash in migrations when adding order_with_respect_to to non-empty table.
Fixed #16028 -- Moved defaultfilters tests into template_tests.
Fixed #23975 -- Restored pre_migrate signal if all apps have migrations.

Thanks kmmbvnr for the report.
Fixed the formatting of one section of the security page
Fixed #24002 -- GenericRelation filtering targets related model's pk

Previously Publisher.objects.filter(book=val) would target
book.object_id if book is a GenericRelation. This is inconsistent to
filtering over reverse foreign key relations, where the target is the
related model's primary key.
Fixed display of lists after website redesign

Thanks Brian Jacobel for the report. refs django/djangoproject.com#197
Fixed #23497 -- Made admin system checks run for custom AdminSites.
Removed netCDF from GIS driver testing; refs #23804.

It may not be installed on all systems.
Fixed #24007 -- Ensure apps registry's ready before unpickling models

This prevents AppRegistryNotReady errors when unpickling Django
models from an external script.
Updated TemplateResponseMixin docs to reflect new behavior after refs #23789.
Allowed Python executable names other than python in test from refs #24007.
Fixed #24014 -- Unified OGRException and GDALException

Thanks Tim Graham for the review.
Refs #24020 -- return expressions from get_group_by_cols()