Synchronize CSSErrorHandler#ignore in HudsonTestCase with the primary version in JenkinsRule

[basil]

This PR does not attempt to solve JENKINS-14749 (which I think was solved but regressed at some point), but it does synchronize the code in HudsonTestCase with that in JenkinsRule by favoring the version in JenkinsRule and applying that to HudsonTestCase. Once synchronized in this PR, the two versions should be kept in sync going forward.

Testing done

Will rely on the CI build.

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