fix(react-webpack): use .jsx extension for JSX test

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: react/webpack example breaks on JSX syntax in a .js file #103
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Node versioning was a bit of a red herring: the real issue was that a .js file, app.spec.js, was using JSX syntax. This renames all files with JSX syntax from .js to .jsx.

Additional bonus fix: opn-cli is deprecated and replaced by open.

[JoshuaKGoldberg]

Other test failures are not from this example: #109, #110.