Bump flat, @open-wc/testing and mocha in /packages/preact-custom-element

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependencies [flat](https://github.com/hughsk/flat), [@open-wc/testing](https://github.com/open-wc/open-wc/tree/HEAD/packages/testing) and [mocha](https://github.com/mochajs/mocha). These dependencies need to be updated together.


Updates `flat` from 4.1.0 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](hughsk/flat@4.1.0...5.0.2)

Updates `@open-wc/testing` from 2.5.25 to 3.1.7
- [Release notes](https://github.com/open-wc/open-wc/releases)
- [Changelog](https://github.com/open-wc/open-wc/blob/master/packages/testing/CHANGELOG.md)
- [Commits](https://github.com/open-wc/open-wc/commits/@open-wc/[email protected]/packages/testing)

Updates `mocha` from 8.1.3 to 8.4.0
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](mochajs/mocha@v8.1.3...v8.4.0)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: "@open-wc/testing"
  dependency-type: direct:development
- dependency-name: mocha
  dependency-type: indirect
...

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