jest-worker's test do not pass when run in worker-farm

[SimenB]

Do you want to request a feature or report a bug?
Either code bug or infrastructure bug

What is the current behavior?
packages/jest-worker/src/__tests__/child.test.js does not pass when doing a full test run locally. It passes on CI as that's run using --runInBand. It also passes locally if you just run that test.

If the current behavior is a bug, please provide the steps to reproduce and either a repl.it demo through https://repl.it/languages/jest or a minimal repository on GitHub that we can yarn install and yarn test.
On current jest master (7db15f0)

$ ./jest worker --no-cache
 PASS  packages/jest-worker/src/__tests__/worker.test.js
 FAIL  packages/jest-worker/src/__tests__/child.test.js
  ● lazily requires the file

    TypeError: Cannot read property 'concat' of undefined

      at handle (node_modules/worker-farm/lib/child/index.js:44:24)

 PASS  packages/jest-worker/src/__tests__/index.test.js
 PASS  packages/jest-haste-map/src/__tests__/worker.test.js
 PASS  packages/jest-config/src/__tests__/get_max_workers.test.js
 PASS  packages/jest-cli/src/reporters/__tests__/coverage_worker.test.js

Test Suites: 1 failed, 5 passed, 6 total
Tests:       1 failed, 41 passed, 42 total
Snapshots:   0 total
Time:        4.12s
Ran all test suites matching /worker/i.

This is also reported in #4630

/cc @mjesun as you wrote the test.

What is the expected behavior?
Test should pass

Please provide your exact Jest configuration and mention your Jest, node, yarn/npm version and operating system.
jest@7db15f0, [email protected]/[email protected]/[email protected], [email protected], macOS High Sierra

[SimenB]

A fix might be to drop worker-farm completely for jest-worker?

Line throwing in worker-farm: https://github.com/rvagg/node-worker-farm/blob/1ee4adb56f7c78eb1f840d58586194e0a4c7474a/lib/child/index.js#L44

[mjesun]

Yeah, this bug is really weird. It looks that the child processes executing the tests get messed up with something; probably process.send. I'm fine deleting the test until we find out what it is; I will get some time to work on that next week.

[mjesun]

Fixed in #5027.

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