chore(deps): bump postcss and react-scripts in /tests/test-app

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.38 and updates ancestor dependency react-scripts. These dependencies need to be updated together.


Updates `postcss` from 7.0.35 to 8.4.38
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.35...8.4.38)

Updates `react-scripts` from 3.4.0 to 5.0.1

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: react-scripts
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>