[UDFs] Abilit to _directly_ unit test function nodes

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

It should be possible to unit test function nodes directly instead of having to test a model that uses the function to test the function node.

Acceptance criteria

A unit test can be written that references a function directly instead of a model

Suggested Tests

A test that has a unit test for a function

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response