Skipped some broken tests on Oracle GIS; refs #23504.
Avoided a crash on an Oracle GIS test; refs #23504.
Added a feature flag for add_srs_entry support; refs #22632 and #23504.
Added feature flag for geometry_field_introspection; refs #22632 and #23504.
Improved GIS migration test cleanup; refs #23504.
Fixed #23488 -- Added AnonymousUser.get_username().
Removed unnecessary order_by() from a GIS test that crashed Oracle.

Oracle cannot order_by() a TextField; refs #23504.
Fixed Oracle GIS failures introduced by e9103402c0; refs #18757.