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

### 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