Add integration tests for ephemeral models rendered as empty operators

dev/dags/dbt/altered_jaffle_shop/models/ephemeral_customers.sql

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+{{ config(materialized='ephemeral') }}
+
+-- An ephemeral model: inlined as a CTE into downstream models and never written to the warehouse.
+select * from {{ ref('customers') }}

dev/dags/dbt/altered_jaffle_shop/models/ephemeral_customers_downstream.sql

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+-- Downstream of the ephemeral model. dbt inlines ephemeral_customers as a CTE here, so the
+-- customers -> ephemeral_customers -> ephemeral_customers_downstream lineage flows through the
+-- ephemeral node even though it is never materialized in the warehouse.
+select * from {{ ref('ephemeral_customers') }}