Jest multi-project-runner still cannot handle exactly one project

[kenrick95]

🐛 Bug Report

I saw #4117 and #7496, which said that it should be fixed in #7498 (released in Jest 24.8.0), but apparently not in my use case where projects is set in root's jest.config.js.

To Reproduce

Steps to reproduce the behavior:

See this repository

---

At the root's jest.config.js:

Specifying projects as only ['packages/p1'] makes "jest-config" fails to read packages/p1/jest.config.js

> yarn test --showConfig
{
  "configs": [
    {
      ..
      "name": "c8776339e2f916fd188945055cabc504",
      ..
     }
  ],
  "globalConfig": {
    ..
    "projects": [
      "packages/p1"
    ],
  }
}

But if I specify projects as ['packages/p1', 'packages/p2'], both configs are successfully read

> yarn test --showConfig
{
  "configs": [
    {
      ..
      "name": "p1",
      ..
     },
    {
      ..
      "name": "p2",
      ..
     }
  ],
  "globalConfig": {
    ..
    "projects": [
      "packages/p1",
      "packages/p2"
    ],
  }
}

Expected behavior

"jest-config" correctly read the configs when there is only one project specified in root's jest.config.js

Link to repl or repo (highly encouraged)

https://github.com/kenrick95/jest-multi-project-runner-config-repro

envinfo

  System:
    OS: macOS 10.14.6
    CPU: (8) x64 Intel(R) Core(TM) i7-4980HQ CPU @ 2.80GHz
  Binaries:
    Node: 12.5.0 - /usr/local/bin/node
    Yarn: 1.16.0 - /usr/local/bin/yarn
    npm: 6.9.0 - /usr/local/bin/npm
  npmPackages:
    jest: ^24.9.0 => 24.9.0

[SimenB]

#8894

[github-actions]

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