[CT-2269] Add schema_string_type fixture to BaseConstraintsColumnsEqual

[MichelleArk]

Currently, test__constraints_wrong_column_data_types assumes the error message type and schema type for an adapter's string type are always the same. On some adapters (like spark!) this is not the case (e.g. valid schema.yml data_type is 'string', error message is 'STR' or 'STRING_TYPE' .

This makes it difficult to test string types themselves in adapter overrides of this method.

To fix this, add an overridable schema_string_type fixture to BaseConstraintsColumnsEqual (can default to string_type), so that it can be overwritten in adapters that have variation between the string schema type and error message. schema_string_type should be used to set wrong_schema_data_type in BaseConstraintsColumnsEqual.test__constraints_wrong_column_data_types.