Fix WebClient missing polyfill

[janfaracik]

Thanks to @basil for raising awareness of this - jenkinsci/jenkins#9148 (comment).

We're now using the JavaScript fetch() API for the builds widget which requires a polyfill for HtmlUnit. This polyfill is enabled by default as part of the Jenkins test harness createWebClient(), however in this case that method wasn't used as a couple of methods of WebClient needed to be overridden.

This MR enables the polyfill for that web client.

Testing done

• Test passes on latest snapshot

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