This predicate is only used in `query_attribute`, and is relatively easy
to remove without adding a bunch of is a checks.

Remaining are `limit`, `precision`, `scale`, and `type` (the symbol
version). These will remain on the column, since they mirror the options
to the `column` method in the schema definition DSL

`.` is regexp meta character. It should be escape for `assert_match`
correctly.

Slightly refactoring `PostgreSQLColumn`. `array` should be readonly.
`default_function` should be initialized by `super`. `sql_type` has been
removed `[]`. Since we already choose to remove it we should not change.

If the test is interrupted in a way that the teardown block fails to
run, the tests will fail to run until the table is removed manually
without this option.

This was only used for uniqueness validations. The first usage was in
conjunction with `limit`. Types which cast to string, but are not
considered text cannot have a limit. The second case was only with an
explicit `:case_sensitive => true` option given by the user.

If we want to have type decorators mess with the attribute, but not the
column, we need to stop type casting on the column. Where possible, we
changed the tests to test the value of `column_defaults`, which is
public API. `Column#default` is not.

In some cases there is a difference between the two, we should always
be doing one or the other. For convenience, `type_cast` is still a
private method on type, so new types that do not need different behavior
don't need to implement two methods, but it has been moved to private so
it cannot be used accidentally.

Revert "test pg, remove unused column assignments. Follow up to 254cdf47"

Related to #15492

This reverts commit 254cdf47.
This reverts commit 4bcf9029.

There are public methods that assume `Column#default` is type casted.
The return value of `Column#default` is publicly relevant and should not change.

/cc @sgrif

- Added assertions about the column. Specifically scale.
- Move record insertion from setup into test method.