Fix typo of failure_percentage in test_stats.py

[phil-davis]

This should have no real impact. It is just an array key that is "internal" to some test code.

[codecov]

Codecov Report

Merging #1511 into master will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1511      +/-   ##
==========================================
- Coverage   81.34%   81.13%   -0.21%     
==========================================
  Files          27       27              
  Lines        2391     2391              
  Branches      368      368              
==========================================
- Hits         1945     1940       -5     
- Misses        353      356       +3     
- Partials       93       95       +2     

Impacted Files	Coverage Δ
locust/clients.py	90.19% <0.00%> (-4.91%)	⬇️

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