Bump @web/test-runner from 0.19.0 to 0.20.0

Bumps [@web/test-runner](https://github.com/modernweb-dev/web/tree/HEAD/packages/test-runner) from 0.19.0 to 0.20.0.
- [Release notes](https://github.com/modernweb-dev/web/releases)
- [Changelog](https://github.com/modernweb-dev/web/blob/master/packages/test-runner/CHANGELOG.md)
- [Commits](https://github.com/modernweb-dev/web/commits/@web/[email protected]/packages/test-runner)

---
updated-dependencies:
- dependency-name: "@web/test-runner"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]