Make infinite run time the default when running headless

[cyberw]

No description provided.

[codecov]

Codecov Report

Merging #1625 (2842ab2) into master (0ca66dd) will decrease coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1625      +/-   ##
==========================================
- Coverage   82.69%   82.59%   -0.11%     
==========================================
  Files          28       28              
  Lines        2566     2568       +2     
  Branches      390      391       +1     
==========================================
- Hits         2122     2121       -1     
- Misses        350      352       +2     
- Partials       94       95       +1     

Impacted Files	Coverage Δ
locust/argument_parser.py	77.19% <ø> (ø)
locust/main.py	20.90% <0.00%> (-0.20%)	⬇️
locust/runners.py	85.57% <0.00%> (-0.39%)	⬇️
locust/user/task.py	96.80% <0.00%> (+0.53%)	⬆️

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