[CT-3291] [Feature] unit test models that depend on external nodes

[graciegoheen]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

You should be able to unit test models that depend on external nodes!

unit_tests:
  - name: test_is_valid_email_address # this is the unique name of the test
    model: my_model # name of the model I'm unit testing
    given: 
      - input: ref('dougs_project', 'users')
        rows:
         - {user_id: 1, email: cool@example.com,     email_top_level_domain: example.com}
         - {user_id: 2, email: cool@unknown.com,     email_top_level_domain: unknown.com}
         - {user_id: 3, email: badgmail.com,         email_top_level_domain: gmail.com}
         - {user_id: 4, email: missingdot@gmailcom,  email_top_level_domain: gmail.com}
      - input: ref('top_level_domains')
        rows:
         - {tld: example.com}
         - {tld: gmail.com}
    expect: # required: the expected output given the inputs above
      - {user_id: 1, is_valid_email_address: true}
      - {user_id: 2, is_valid_email_address: false}
      - {user_id: 3, is_valid_email_address: false}
      - {user_id: 4, is_valid_email_address: false}

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[MichelleArk]

This could be tested similar to what's done for testing partial parsing with external nodes here

[graciegoheen]

Notes from estimation:

• This may already work, but we should add a test and add more defense code for how we're handling these node attributes