https://gerrit.wikimedia.org/r/#/c/406633/ introduced a browser test that is not compatible with the Echo extension and thus the Minerva skin.
Echo magically moves the usermessage div and when the browser test runs it cannot find the element in the page.
All patches in Minerva and Echo are currently rejected by Jenkins (and we can't override verified).
e.g. https://integration.wikimedia.org/ci/job/mediawiki-core-qunit-selenium-jessie/13853/console on patch https://gerrit.wikimedia.org/r/#/c/406618/
It seems strange as the screenshot for the failure shows that the element is present in the page so it's unclear why this fails from the information provided.
Unrelated to Jenkins failure but worth considering:
It's also not clear why this browser test is in core given that the usermessage div is created by skins. Some skins do not make use of it, notably Minerva.