Support maxWorkers or runInBand when using 'projects'

Bug Report

When defining multiple projects via the projects config option, where one project has maxWorkers set (ie. 1), that setting is not respected.

In the case of a mono-repo, assume we have two projects. One which runs parallel, and another which needs to be run sequentially. For this, the latter has the config setting maxWorkers: 1.

When we run jest from the base config, however, the latter runs in parallel, ignoring the maxWorkers: 1 setting.

To Reproduce

1. Setup a mono-repo with two jest config files
2. Set one to maxWorkers: 1
3. Add both config paths to base projects property
4. Run jest

Expected behavior / Proposal

I propose the following:

1. Determine global max workers as:
   • (If present) maxWorkers setting in base config
   • (Otherwise) Determine the maximum value specified for maxWorkers found in all projects config files
2. Allow each project to use
   • (If present) Its own maxWorkers # of workers
   • (Otherwise) The global maximum # of workers

Effectively, this ensures that maxWorkers behaviour matches that of running jest on an individual project.

envinfo

  System:
    OS: Windows 10 10.0.19041
    CPU: (4) x64 Intel(R) Core(TM) i7-4510U CPU @ 2.00GHz
  Binaries:
    Node: 14.13.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.4 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.14.4 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    jest: ^26.5.3 => 26.6.2