According to the build log, master build #301 had a failing storybook test (later fixed by https://github.com/zanata/zanata-platform/commit/a874be9a93da0e1d1a294c60d0913df0363651a6) but was still considered a successful build.
You can trigger a similar test failure by adding this:
expect("THIS SHOULD FAIL").toEqual("something else")
to one of the tests, eg server/zanata-frontend/src/frontend/app/components/DraggableVersionPanels/DraggableVersionPanels.test.js
Ultimately, it would be best to have Jest output a file format which Jenkins can parse (perhaps by switching to the Jenkins XUnit Plugin, or perhaps by having Jest output junit.xml), but for now we should just ensure that failing tests actually fail the build.