Fix stopping and restarting of LoadTestShape test

[max-rocket-internet]

Resolves #1540

Reset state when stopping a LoadTestShape test and also add a test for this.

[codecov]

Codecov Report

Merging #1541 into master will decrease coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
- Coverage   82.80%   82.54%   -0.26%     
==========================================
  Files          28       28              
  Lines        2564     2566       +2     
  Branches      390      391       +1     
==========================================
- Hits         2123     2118       -5     
- Misses        345      350       +5     
- Partials       96       98       +2     

Impacted Files	Coverage Δ
locust/runners.py	83.87% <100.00%> (-0.90%)	⬇️
locust/user/task.py	97.29% <0.00%> (-0.55%)	⬇️
locust/stats.py	90.68% <0.00%> (-0.21%)	⬇️

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[cyberw]

👍