Add more tests

[pankajastro]

closes: #199

• Add test for the outlets param and load YML dag with default suffix cases
• Cleanup some import for Airflow < 2.0.0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.53%. Comparing base (79bc648) to head (70e029e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #258      +/-   ##
==========================================
+ Coverage   89.35%   92.53%   +3.18%     
==========================================
  Files           8        8              
  Lines         648      630      -18     
==========================================
+ Hits          579      583       +4     
+ Misses         69       47      -22     

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[tatiana]

Thanks, @pankajastro , a minor comment, and we're good to merge this!