Expose jest's slowTestThreshold option

[dargue3]

Hello CRA, thanks for bringing the new update :)

I often do large integration tests in lieu of unit testing, and as such, test suites that test an entire page often take around 15 seconds.

Jest 26.2 released a feature called slowTestThreshold where you could set a threshold for when jest would highlight the run time of your tests as red. View the docs here

This option is unfortunately not green-lighted by CRA's out of the box jest setup. I would love to have this as an option exposed from CRA's package.json 🙏 If this is something that's first-issue-level and I was pointed in the right direction, I could manage making a PR.