input csv file support on 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: 45 lines in your changes are missing coverage. Please review.

Comparison is base (f629baa) 64.65% compared to head (922c1af) 83.46%.
Report is 9 commits behind head on unit_testing_feature_branch.

Additional details and impacted files

@@                       Coverage Diff                        @@
##           unit_testing_feature_branch    #9035       +/-   ##
================================================================
+ Coverage                        64.65%   83.46%   +18.80%     
================================================================
  Files                              181      182        +1     
  Lines                            26973    27098      +125     
================================================================
+ Hits                             17440    22617     +5177     
+ Misses                            9533     4481     -5052     

Flag	Coverage Δ
integration	83.46% <60.17%> (?)
unit	?

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

Files	Coverage Δ
core/dbt/compilation.py	95.94% <100.00%> (+37.83%)	⬆️
core/dbt/config/project.py	94.10% <100.00%> (+3.81%)	⬆️
core/dbt/context/providers.py	89.48% <100.00%> (+25.29%)	⬆️
core/dbt/contracts/files.py	94.61% <100.00%> (+18.94%)	⬆️
core/dbt/node_types.py	100.00% <100.00%> (+12.72%)	⬆️
core/dbt/parser/read_files.py	84.50% <ø> (+38.49%)	⬆️
core/dbt/context/context_config.py	93.10% <50.00%> (+13.20%)	⬆️
core/dbt/contracts/graph/model_config.py	89.80% <70.00%> (+6.38%)	⬆️
core/dbt/contracts/graph/unparsed.py	93.56% <85.00%> (+6.79%)	⬆️
core/dbt/contracts/graph/nodes.py	89.05% <63.82%> (+11.29%)	⬆️
... and 1 more

... and 133 files with indirect coverage changes

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