Improving unit test coverage over DRY support

[Ceddaerrix]

What does this PR do?

• This PR is about introducing the unit test that was discarded under PR 929 assuming some of the limitations of the matchRegex assert in Helm-unittest

If you modified files in the ./charts/jenkins/ directory, please also include the following:

### Submitter checklist
- [x] I bumped the "version" key in `./charts/jenkins/Chart.yml`.
- [x] I followed the [technical requirements](https://github.com/jenkinsci/helm-charts/blob/main/CONTRIBUTING.md#technical-requirements).

Special notes for your reviewer