More tests

[na--]

This should close #1873 and adds the missing test from #1882. This can also be considered a somewhat tentative first step in #342

[codecov-io]

Codecov Report

Merging #1885 (30704a3) into master (1721868) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1885      +/-   ##
==========================================
+ Coverage   71.12%   71.22%   +0.09%     
==========================================
  Files         183      183              
  Lines       14319    14319              
==========================================
+ Hits        10185    10198      +13     
+ Misses       3506     3495      -11     
+ Partials      628      626       -2     

Flag	Coverage Δ
ubuntu	71.16% <ø> (+0.07%)	⬆️
windows	70.79% <ø> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
output/json/json.go	84.48% <ø> (ø)
js/runner.go	81.44% <0.00%> (+0.57%)	⬆️
output/cloud/output.go	80.62% <0.00%> (+1.83%)	⬆️
core/engine.go	87.50% <0.00%> (+1.85%)	⬆️

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[mstoykov]

LGTM

Maybe move the test extensions in the samples ?

[na--]

I thought about samples, but these extensions aren't a good example for potential extension authors... so I'd prefer it is very clear that they are for testing purposes only

[imiric]

LGTM

And we want to run this on every commit, right? I suppose it's fine, and it runs quick enough.