Misc fixes for summary callback changes

[na--]

This fixes #1316 and contains the prerequisite changes (and a few minor unrelated ones) for #1768. I've taken the extraneous changes from that PR, split them apart in sensible commits, and added some more tests. Hopefully, the only thing that remains in #1768 would be the actual handleSummary() changes, though I may have a few other minor commits to make here before I push the final version of the other PR (likely tomorrow).

[codecov-io]

Codecov Report

Merging #1788 (10ac39f) into master (ccffbca) will increase coverage by 0.17%.
The diff coverage is 79.71%.

@@            Coverage Diff             @@
##           master    #1788      +/-   ##
==========================================
+ Coverage   71.43%   71.60%   +0.17%     
==========================================
  Files         178      178              
  Lines       13777    13789      +12     
==========================================
+ Hits         9841     9874      +33     
+ Misses       3323     3303      -20     
+ Partials      613      612       -1     

Flag	Coverage Δ
ubuntu	71.54% <79.71%> (+0.13%)	⬆️
windows	70.11% <79.71%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/config.go	88.18% <ø> (+3.26%)	⬆️
cmd/run.go	7.39% <0.00%> (+0.18%)	⬆️
lib/runtime_options.go	100.00% <ø> (ø)
stats/cloud/cloud_easyjson.go	33.97% <0.00%> (ø)
ui/summary.go	89.35% <ø> (ø)
stats/stats.go	77.11% <50.00%> (-1.13%)	⬇️
cmd/runtime_options.go	87.69% <93.33%> (+6.29%)	⬆️
core/engine.go	93.03% <100.00%> (+2.08%)	⬆️
js/bundle.go	90.54% <100.00%> (ø)
lib/models.go	94.11% <100.00%> (+0.17%)	⬆️
... and 4 more

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[mstoykov]

LGTM, just a minor point that we might be degrading the marshalling performance, but someone will have to run/write a benchmark that uses it to see how much it is degrading if at all

[imiric]

👍