sql/ltree: add additional logic tests

As part of the QA effort for LTREE data, add additional logic test
cases:

- remaining comparison operators
- defaults and check constraints
- schema changes
- views and computed columns
- usage in UDFs
- sorting behavior
- use in composite indexes
- type casting edge cases and boundary conditions

Informs: cockroachdb#44657
Release notes: none

Author: mw5h