Make certain that even sneakily open can't be used outside the init c…

[mstoykov]

…ontext

[codecov-commenter]

Codecov Report

Merging #1475 into new-schedulers will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           new-schedulers    #1475      +/-   ##
==================================================
+ Coverage           76.69%   76.71%   +0.01%     
==================================================
  Files                 163      163              
  Lines               13011    13010       -1     
==================================================
+ Hits                 9979     9980       +1     
+ Misses               2520     2518       -2     
  Partials              512      512              

Impacted Files	Coverage Δ
js/initcontext.go	90.42% <100.00%> (+0.20%)	⬆️
js/runner.go	82.50% <100.00%> (-0.19%)	⬇️
lib/executor/variable_looping_vus.go	94.46% <0.00%> (-0.80%)	⬇️
lib/executor/vu_handle.go	97.01% <0.00%> (+2.98%)	⬆️
lib/helpers.go	100.00% <0.00%> (+4.16%)	⬆️

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[na--]

LGTM in general