support csv file fixtures for unit tests

[emmyoop]

resolves #8290

Problem

No support for csv files in unit testing.

Solution

Support a csv file in addition to an inline csv definition.

  - name: test_my_model_empty
    model: my_model
    given:
      - input: ref('my_model_a')
        format: csv
        fixture: test_my_model_a_empty_fixture_csv
      - input: ref('my_model_b')
        format: csv
        fixture: test_my_model_fixture

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (f629baa) 64.65% compared to head (e8aade6) 85.42%.

Additional details and impacted files

@@                       Coverage Diff                        @@
##           unit_testing_feature_branch    #9014       +/-   ##
================================================================
+ Coverage                        64.65%   85.42%   +20.76%     
================================================================
  Files                              181      181               
  Lines                            26973    26984       +11     
================================================================
+ Hits                             17440    23051     +5611     
+ Misses                            9533     3933     -5600     

Flag	Coverage Δ
integration	82.11% <85.71%> (?)
unit	64.63% <28.57%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/dbt/contracts/graph/unparsed.py	93.94% <85.71%> (+7.17%)	⬆️

... and 117 files with indirect coverage changes

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