Add unit test

[SchrodingersGat]

• Unit test to ensure event is fired when stock item is created OR updated
• Ref: after_save_stock_item #8546
• Ref: Email Notification When Stock Level Drops Below Minimum? #8452

[SchrodingersGat]

@skydiablo for your reference :)

[netlify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.57%. Comparing base (ee9980e) to head (1f465ba).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8550   +/-   ##
=======================================
  Coverage   84.57%   84.57%           
=======================================
  Files        1178     1178           
  Lines       53690    53709   +19     
  Branches     2028     2028           
=======================================
+ Hits        45406    45425   +19     
  Misses       7760     7760           
  Partials      524      524           

Flag	Coverage Δ
backend	85.96% <100.00%> (+<0.01%)	⬆️

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