[UDFs] Functions can be _mocked_ while unit testing a model

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

Instead of actually calling the function, it should be possible to specify the value the function should return for the model being unit tested

Acceptance criteria

It's possible to specify the value the function should return for the model being unit tested

Suggested Tests

Unit test a model using a mocked function

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response