Description

We should allow folks to add unit tests to incremental models

Acceptance criteria

• Ability to test incremental models in both full refresh and incremental mode
• Ability to specify the current status of an incremental model to test the incremental behavior

More on the above two points in #8275 (reply in thread):

To support this, dbt's unit testing framework would need to provide the ability to:

• override the is_incremental macro to return True (p0), and provide the ability to specify an input fixture for {{ this }} (p1) in order to mock out the previous state of an incremental model. The expect output would be the set of rows that dbt's incremental materialization would insert/merge into the existing relation in production, as opposed to the resulting updated representation of the model.

To be moved into a separate issue:

• Ability to test the incremental behavior of models using incremental strategy that requires partitioning (e.g. insert overwrite in BigQuery): [CT-3129] Ability to test the incremental behavior of models after merge/insert/append/etc has occurred #8664 (blocked on spike [CT-3033] [Spike] Explore support multiple unit test materialization strategies: CTE vs 'seed'-based #8499)

A "current_actual" could define the current state of the model we're incrementing on, testing whether the incrementing works as expected.