Change NowcastWorker mock to pytest fixture

[douglatornell]

Test suite maintenance. re: issue #81

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.53%. Comparing base (6db322f) to head (77260b8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
- Coverage   77.71%   77.53%   -0.19%     
==========================================
  Files         132      132              
  Lines       18782    18630     -152     
  Branches      762      762              
==========================================
- Hits        14597    14445     -152     
  Misses       4119     4119              
  Partials       66       66              

Flag	Coverage Δ
unittests	77.53% <100.00%> (-0.19%)	⬇️

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