Fix tests on Jenkins 2.411

[timja]

see jenkinsci/jenkins#8087 (comment)

Testing done

I ran the test that we were notified that was failing.
The above PR adds icon support to buttons which adds some new lines to the label in htmlunit.

It could be refactored to use j.submit(form) but I've just done the smallest fix here.

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[basil]

Thanks for the PR!