Slim down test helpers in config.rs

[larsewi]

Summary

• Drop `make_table_config` — only two callers, the inline struct literal is just as short and self-documents.
• Collapse `make_field` to a two-arg `field(name, primary_key)`. The previous four-arg version had `value_kind` and `null_sentinel` that were never load-bearing for the `ordered_field_names` tests, and the unlabeled positional args made call sites hard to scan.
• Brings these tests in line with the rest of the module, which already uses inline `FieldConfig { ..Default::default() }` literals.