Bump wp-cli/wp-cli-tests from 3.1.8 to 3.1.9

Bumps [wp-cli/wp-cli-tests](https://github.com/wp-cli/wp-cli-tests) from 3.1.8 to 3.1.9.
- [Release notes](https://github.com/wp-cli/wp-cli-tests/releases)
- [Commits](wp-cli/wp-cli-tests@v3.1.8...v3.1.9)

---
updated-dependencies:
- dependency-name: wp-cli/wp-cli-tests
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]