Refactoring configuration test

[sparsick]

Refactoring of configuration tests

jenkinsci/jenkins-test-harness#426

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[jglick]

Looks nicer indeed!

I will merge the upstream PR, which if all goes well should trigger a release of jenkins-test-harness. At that point you can bump the version here and take it out of draft. Or you can wait for Dependabot to offer the new version as a default in plugin-pom, and for that to be merged and at some point released.