[UDFs] Functions resolve properly in unit tests

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

If a model that is being unit tested uses the function macro, then the test should appropriately resolve the function.

Acceptance criteria

function macro can be resolved in models that call them in unit testing.

Suggested Tests

A unit test that tests a model with a function

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response