[CT-2997] Ability to test incremental models

[gshank]

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.

[christopherekfeldt]

We need this! We would love to have the functionality to only test the newly added / updated data for incremental models.

[martynydbt]

@gshank please pull "Ability to test the incremental behavior of models using incremental strategy that requires partitioning (e.g. insert overwrite in BigQuery)" out into a separate ticket

[jtcohen6]

2x2 = 4 acceptance tests for us to write as part of implementation:

• {{ is_incremental() }} : True + False
• {{ this }} fixture: provided or not

Discussion:

• is_incremental would go in existing spec for overrides
• Need to add this to spec for inputs (cc: @graciegoheen @dbeatty10)

[MichelleArk]

Here's how i'm imagining the spec for this:

unit-tests: 
 - model: my_model
   tests: 
    - name: test_name
      given: 
      - input: this
         rows: ...