build(deps-dev): bump @open-wc/testing from 3.1.5 to 3.1.6

Bumps [@open-wc/testing](https://github.com/open-wc/open-wc/tree/HEAD/packages/testing) from 3.1.5 to 3.1.6.
- [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/testing@3.1.6/packages/testing)

---
updated-dependencies:
- dependency-name: "@open-wc/testing"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>