Add maxWorkers to jest config

Created by: akmjenkins

Is your proposal related to a problem?

I doubt this is a new proposal/issue, but we are now experiencing constant CI memory issues

Describe the solution you'd like

I'd like to add maxWorkers to https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/scripts/utils/createJestConfig.js#L78

Describe alternatives you've considered

I've tried setting react-scripts --max_old_space_size=4096 as suggested elsewhere, but it's not working.

Additional context

None.