Add a method to stop a running Engine

[hynd]

Including a new stop cmd to trigger it via the HTTP API.
Closes #1352.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #1355 into new-schedulers will decrease coverage by 0.00%.
The diff coverage is 36.84%.

@@                Coverage Diff                 @@
##           new-schedulers    #1355      +/-   ##
==================================================
- Coverage           76.60%   76.59%   -0.01%     
==================================================
  Files                 160      160              
  Lines               12567    12584      +17     
==================================================
+ Hits                 9627     9639      +12     
- Misses               2431     2436       +5     
  Partials              509      509              

Impacted Files	Coverage Δ
api/v1/status_routes.go	10.20% <0.00%> (-0.67%)	⬇️
core/engine.go	91.45% <76.47%> (-1.56%)	⬇️
api/v1/status.go	100.00% <100.00%> (ø)
lib/helpers.go	100.00% <0.00%> (+4.16%)	⬆️

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[na--]

@hynd, to fix these linter issues locally, take a look at the "Running the linter" section in https://github.com/loadimpact/k6/blob/master/CONTRIBUTING.md#development-setup

Also, I realize that a k6 stop command would be convenient, but I'm not quite convinced by its necessity. I don't mind if we add a new minor REST API method, but a new-subcommand requires some more consideration...

[hynd]

Understood - the stop command wasn't something i needed, just seemed quick to implement :)
I've removed it and fixed up the CI items.

[na--]

LGTM

[na--]

Thanks!