test(dags): Add tests for dbt operators in DAGs

[tomasfarias]

Closes #55

This PR adds more integral testing by defining DAGs with Dbt* operators, triggering runs, and checking outputs. We test both the context manager way of defining a dag as well as the TaskFlow API.

As I was working on this I noticed the example DAGs were actually broken 😅. So this PR also fixes them.

[tomasfarias]

We should add a test that includes a on_failure_callback default parameter and a DbtBuildOperator task to see if we can catch #51

[codecov]

Codecov Report

Merging #56 (dac0760) into master (c4d13d3) will increase coverage by 0.10%.
The diff coverage is n/a.

❗ Current head dac0760 differs from pull request most recent head 7806761. Consider uploading reports for the commit 7806761 to get more accurate results

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   98.58%   98.69%   +0.10%     
==========================================
  Files           8        8              
  Lines         917      917              
==========================================
+ Hits          904      905       +1     
+ Misses         13       12       -1     

Flag	Coverage Δ
unittests	98.69% <ø> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
airflow_dbt_python/operators/dbt.py	100.00% <0.00%> (+0.33%)	⬆️

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[tomasfarias]

Unfortunately, dag tests interact with a lot of Airflow internals that have changed even from version 2.0 to 2.2. Maintaining all 3 versions of Airflow for that set of tests will be too hard, so the older two have been dropped.

Will update the docs to mention examples have been tested with Airflow > 2.2