Add configurable setup and teardown timeouts

[na--]

This fixes #576, though we should add some upper limits when implementing #539

[codecov-io]

Codecov Report

Merging #602 into master will increase coverage by 0.04%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #602      +/-   ##
==========================================
+ Coverage   61.99%   62.03%   +0.04%     
==========================================
  Files          97       97              
  Lines        7470     7478       +8     
==========================================
+ Hits         4631     4639       +8     
+ Misses       2587     2585       -2     
- Partials      252      254       +2

Impacted Files	Coverage Δ
lib/options.go	98.3% <ø> (ø)	⬆️
cmd/options.go	37.63% <0%> (-0.83%)	⬇️
core/local/local.go	81.09% <100%> (+0.39%)	⬆️
stats/stats.go	39.18% <0%> (ø)	⬆️
js/modules/k6/metrics/metrics.go	75% <0%> (ø)	⬆️
core/engine.go	89.83% <0%> (+1.06%)	⬆️

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