Split TestRunner off of TestScheduler

[cpojer]

Summary

Second step in making test runners pluggable. This simply splits up the big scheduler class into the TestScheduler that instruments everything and a concrete TestRunner that knows how to schedule JavaScript tests.

Test plan

jest

[boujeepossum]

seems like TestRunner doesn't even need a state, we can do

import {runTests} from './test_runner';

// ...

runTests(globalConfig, ...restOfArguments)

[cpojer]

This line is temporary and will change very soon.

[codecov-io]

Codecov Report

Merging #4233 into master will decrease coverage by 0.01%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master    #4233      +/-   ##
==========================================
- Coverage   60.13%   60.12%   -0.02%     
==========================================
  Files         191      192       +1     
  Lines        6756     6761       +5     
  Branches        6        6              
==========================================
+ Hits         4063     4065       +2     
- Misses       2690     2693       +3     
  Partials        3        3

Impacted Files	Coverage Δ
packages/jest-cli/src/test_scheduler.js	20.58% <40%> (-7.88%)	⬇️
packages/jest-cli/src/test_runner.js	55% <55%> (ø)
packages/jest-cli/src/reporter_dispatcher.js	29.41% <0%> (-11.77%)	⬇️

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[github-actions]

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