Introduce UI tests

[strangelookingnerd]

Following on #210 (comment) this PR aims to introduce UI tests.

While implementing those tests I noticed that summaries without an icon are currently "omitted" without any notice. As the jelly tag summary that we use requires a value I added a fallback that will return the emtpy.png from jenkins core alongside logging a warning. This allows us to create "text-only" summaries.

Testing done

mvn clean verify

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