Bump jenkins-test-harness

Bumps [jenkins-test-harness](https://github.com/jenkinsci/jenkins-test-harness) from 1706.v5257fc59612a_ to 1712.v68cd645cb_f35.
- [Release notes](https://github.com/jenkinsci/jenkins-test-harness/releases)
- [Changelog](https://github.com/jenkinsci/jenkins-test-harness/blob/master/docs/CHANGELOG-OLD.md)
- [Commits](https://github.com/jenkinsci/jenkins-test-harness/commits

)

---
updated-dependencies:
- dependency-name: org.jenkins-ci.main:jenkins-test-harness
  dependency-type: direct:development
...

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