Move core/local/ExecutionScheduler to execution/Scheduler

[na--]

...and also move and expand the special context wrapper that test.abort() uses to interrupt the test and store the error internally. Its improved capabilities will be required for #1889

This is built on top of #2810 and is also the first step towards addressing #1302. There is still a long way to go before we can move all of the execution-related things out of lib/, mostly because it will be quite tricky to both have a nice module structure and avoid import loops 😞 Even now, parts of execution/ (esp. the tests) still import lib/executor/, while lib/executor imports the parts of lib/ we want to move to execution/... 😭 I worked around the issue this time, though that might not always work as well as it did this time.

In any case, I just made this change to get the process of cleaning up lib/ started. Between this PR and the next one to address #1889, it's much easier to remove the core/ package than it is to cleanly split apart lib/. Now, we have at least a place for these things, even if we can't move everything quite yet.

[codecov-commenter]

Codecov Report

Merging #2812 (08d5acf) into master (a2a5b39) will decrease coverage by 0.35%.
The diff coverage is 77.14%.

❗ Current head 08d5acf differs from pull request most recent head a42bdc7. Consider uploading reports for the commit a42bdc7 to get more accurate results

@@            Coverage Diff             @@
##           master    #2812      +/-   ##
==========================================
- Coverage   76.88%   76.54%   -0.35%     
==========================================
  Files         218      217       -1     
  Lines       16650    16664      +14     
==========================================
- Hits        12802    12755      -47     
- Misses       3041     3092      +51     
- Partials      807      817      +10     

Flag	Coverage Δ
ubuntu	?
windows	76.54% <77.14%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
api/v1/status_routes.go	31.25% <ø> (-27.94%)	⬇️
core/engine.go	94.33% <ø> (ø)
lib/execution.go	87.15% <ø> (-2.76%)	⬇️
execution/abort.go	70.37% <70.37%> (ø)
cmd/run.go	72.22% <100.00%> (ø)
execution/scheduler.go	90.07% <100.00%> (ø)
lib/executor/helpers.go	100.00% <100.00%> (+0.97%)	⬆️
lib/netext/tls.go	27.08% <0.00%> (-22.92%)	⬇️
api/v1/metric.go	54.54% <0.00%> (-13.64%)	⬇️
... and 11 more

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[oleiade]

Very good work 👍🏻 I'm glad to see the move to a dedicated execution package happen 🎉 🦖 I've left a suggestion to maybe find a more suiting name for the Scheduler as we're at it, but I've really no strong opinion on this, and you should feel free to walk past it 🙇🏻

[codebien]

LGTM, just a minor nitpick