drupalLogin in functional JavaScript tests passes locally but fails on Drupal.org?
After reading through the Jenkins results for the nth time, it finally occurred to me that during Drupal core (and other contrib) tests all JavaScript tests always run separately from the other tests. All my tests however were in the same list. 

This happened because all test classes started originally as functional tests, but as I went ahead some of them were changed into functional javascript tests. Class extending was changed into JavascriptTestBase and the tests were working perfectly fine. At least, when running individually. Running multiple tests at the same time made them somehow interfere with one another.

The problem was that I had never changed the namespace from Functional into FunctionalJavascript. The Drupal testrunner makes sure that all FunctionalJavascript tests are ran one by one, but performs other tests in parallel. 

After changing the namespace, the tests were listed separate from the rest, ran after each other and finally showed up green on Drupal.org