Fix rn-tester convenience scripts (#49401)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49401

These scripts erroneously invoked `npm`, requiring `--` to forward args.

Discovered while testing https://github.com/facebook/react-native/pull/49353.

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D69594024

fbshipit-source-id: a6e8cccaf994f1472b51155d36a49ed17cd291dc