Introduce execution.Controller with a local no-op implementation

[na--]

What?

This is another sliver of the newly refactored #2816, a follow-up to #3191.

It adds the concept of an execution.Controller, a very lightweight API by which multiple k6 instances could synchronize the concurrent execution of different segments of the same test run. However, it doesn't actually add any distributed execution by itself, just a local implementation of it that doesn't do anything (because a single instance doesn't need to synchronize with itself... yet 😅)

Why?

This is a backwards-compatible prerequisite for distributed execution.

As you can see, no tests have been changed beyond adding the extra parameter to execution.NewScheduler(). So this doesn't change any existing k6 behavior. However, it makes merging the distributed execution much easier because it reduces the likelihood of conflicts. For example, I recently had to resolve conflicts between #2816 and #3112. Even ignoring the code conflicts, logical conflicts are also easier to foresee when this is merged into the codebase, and it makes distributed execution a smaller and easier to review PR.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes. (not needed)
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

• Clustering and distributed execution #140
• Distributed Execution Proposal #3218
• [WIP] Proof of concept changes (test suites, HDR histograms) #2816
• Add a design doc for distributed execution and test suites #3217
• Test suites / execute multiple scripts with k6 #1342

[codecov-commenter]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (1fe0563) 73.19% compared to head (3aa520b) 73.16%.

❗ Current head 3aa520b differs from pull request most recent head fd81209. Consider uploading reports for the commit fd81209 to get more accurate results

Files	Patch %	Lines
execution/scheduler.go	57.57%	7 Missing and 7 partials ⚠️
execution/controller.go	75.00%	1 Missing and 1 partial ⚠️
cmd/inspect.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3204      +/-   ##
==========================================
- Coverage   73.19%   73.16%   -0.03%     
==========================================
  Files         267      261       -6     
  Lines       20076    19910     -166     
==========================================
- Hits        14694    14567     -127     
+ Misses       4468     4413      -55     
- Partials      914      930      +16     

Flag	Coverage Δ
ubuntu	73.09% <69.64%> (-0.04%)	⬇️
windows	73.01% <69.64%> (-0.04%)	⬇️

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[na--]

Applied PR review suggestion and rebased on top of the latest master

[olegbespalov]

🚀

[na--]

🎉