Fix run-ci-javascript-tests script (#52364)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52364

Changelog: [Internal]

D76512374 broke error reporting in `run-ci-javascript-tests.js`. This is partly because the file was untyped and we missed that the `.code` check on the result of `execSync` was always going to be falsy. Also, `execSync`'s default error handling mechanism is not human-friendly - it throws an `Error` with `Buffer`s for stdout and stderr (see [example](https://github.com/facebook/react-native/actions/runs/16003656383/job/45144825919?pr=52357&fbclid=IwY2xjawLRyfpleHRuA2FlbQIxMQBicmlkETFZSG1xeWhTWWczR1paS0lKAR4pF46Z-J2CbSk7YdHZJ-N3F9eQJ7hR4EowfLV6mUtzMLg8j-EWdZiGY1la6A_aem_1Zbvn6fD5NS9YO-B7QJssg)).

Here, I'm adding types, removing dead code and preserving stdout and stderr from all child processes in a human-readable format.

Reviewed By: huntie

Differential Revision: D77648312

fbshipit-source-id: c6d98e668d6edf15729fa02fecb3408b9dd6debc