[CT-2918] Implement inline fixture reuse, in a "fixtures" yaml file section

[gshank]

Description

Enable reuse of fixtures in unit tests.

fixtures:
 - name: my_model_a_with_nulls
   rows: 
    - {id: 1, b: 2}
    - {id: null, b: null}
unit: 
 - model: my_model
   tests: 
    - name: test_name
      given: 
      - input: ref('my_model_a')
        fixture: my_model_a_with_nulls

Acceptance Criteria

Inline csv input works in unit tests.

Impact to other teams

None

Will backports be required?

No

Context