docs: Document --testFailureExitCode

[ArkadyDR]

Summary

Includes the --testFailureExitCode config parameter in the documentation. This parameter was added in the 21.0.0 release by #4067 but wasn't included in the release notes or added to the documentation.

From a motivation perspective, at my company we're incorporating a variety of small create-react-app's into a much larger product that has a handrolled continuous integration system. It considers a non-zero exit code of a testing tool to be a build environment failure and will abort subsequent steps (it scoops up the test reports and process them to determine test failures). Given Jest's default behaviour of returning 1 for both test failures and tool failures I was going to need to find a way to distinguish between a test and a tool failure. I tried searching for this exact configuration in the docs and generally on google and came up empty, and then randomly stumbled on this CLI/config parameter when playing with a custom test runner (the example logs the whole config object to the console).

It'd be nice for this to be included in the official documentation so (a) others could benefit and (b) we had some confidence that it's not going to silently dissapear or become broken.

I've backdated it through the past versions as it was added before the oldest version of the documentation currently on the website.

Test plan

Ran up the website following contributor instructions (yarn, yarn start in the website directory). Item appears in the TOC and renders correctly.

[jeysal]

Looks like this was added quite a long time ago #4067 probably because of a FB need, but it makes sense for this to be public to avoid people having to grep output or something, so good to have it documented.

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