Flaky test fixes part2

[mstoykov]

One more batch of changes for #1357

[codecov-commenter]

Codecov Report

Merging #1568 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1568      +/-   ##
==========================================
- Coverage   77.11%   77.09%   -0.02%     
==========================================
  Files         162      162              
  Lines       13192    13192              
==========================================
- Hits        10173    10171       -2     
- Misses       2499     2501       +2     
  Partials      520      520              

Impacted Files	Coverage Δ
lib/executor/vu_handle.go	95.49% <0.00%> (-0.91%)	⬇️
lib/executor/constant_arrival_rate.go	96.75% <0.00%> (-0.55%)	⬇️

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[imiric]

It's difficult to tell from timing changes whether this is a definitive fix 😄

We might need some refactoring in how they work and maybe trying one of the time mocking packages after all. 😕