Fix bug causing test_stop event to be fired twice in master node

[heyman]

Fix for #1638

[codecov]

Codecov Report

Merging #1641 (15c5ddc) into master (12ff1d8) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1641      +/-   ##
==========================================
+ Coverage   80.53%   80.82%   +0.29%     
==========================================
  Files          30       30              
  Lines        2677     2676       -1     
  Branches      411      410       -1     
==========================================
+ Hits         2156     2163       +7     
+ Misses        424      419       -5     
+ Partials       97       94       -3     

Impacted Files	Coverage Δ
locust/runners.py	86.12% <100.00%> (+0.93%)	⬆️
locust/stats.py	88.82% <0.00%> (ø)
locust/user/task.py	96.27% <0.00%> (+0.53%)	⬆️
locust/event.py	92.68% <0.00%> (+4.87%)	⬆️

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[cyberw]

nice!