JavaScript folder: rename async-apis, react-js, and testing folders & files within them

[01zulfi]

Complete the following REQUIRED checkboxes:

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text

Complete the following checkboxes ONLY IF they are applicable to your PR. You can complete them later if they are not currently applicable:

• I have previewed all lesson files included in this PR with the Markdown preview tool to ensure the Markdown content is formatted correctly
• I have ensured all lesson files included in this PR follow the Layout Style Guide

---

1. Because:

• Cleaning up folder/file structure in the curriculum repository top-meta#109

2. This PR:

• Renames async-apis, react-js, and testing folders and files within them

3. Additional Information:

n/a