[CT-3038] Support input sources, create test cases

[gshank]

Description

Currently unit tests can take inputs from models: ref('my_model'). We also want to allow sources as inputs: source('source_dataset', 'source_name'). The big difference between these two things is that dbt builds the relations for models, whereas sources already exist as relations, outside of dbt.

Do we want to use the source as-is? This is probably simplest. It might also be possible to construct an ephemeral model which only contains the columns to be tests.

It's possible that this will require different jinja implementation than the current ephemeral model implementation of input models.

Acceptance Criteria

A unit test can reference a source node and "just work".

Impact to other teams

None

Will backports be required?

No

Context