Background: WikibaseLexeme extension adds node selenium tests, and needs them to be part of extension's CI build. As not all of the extension's dependencies are part of mediawiki vendor yet, running the tests require install dependencies "locally" with composer.
In order to achieve the above, we've created a dedicated job (there was no node selenium + composer combo before): mwext-mw-selenium-node-composer-jessie.
With the patch the should be green, i.e. https://gerrit.wikimedia.org/r/#/c/422931/, we're experiencing quite some flakiness of MW core tests (our tests are consistently passing).
Example of a failing job: https://integration.wikimedia.org/ci/job/mwext-mw-selenium-node-composer-jessie/11/console.
As core's node selenium job (mediawiki-core-qunit-selenium-jessie) seem to be rather reliable, it could be there there is some difference in config between mwext-mw-selenium-node-composer-jessie and mediawiki-core-qunit-selenium-jessie. I failed to find the missing bits though. Maybe @zeljkofilipin has an idea where to look at?
Core test failures have also not been reproduced when running tests locally.