In /tests/selenium/specs directory nodejs inbuilt assert module is being used. The project already uses webdriverio that comes with its own assertion library called expect-webdriverio that provides more complex assertions methods as well as are more readable. The task is to change assert to expect. Even the jest directory uses expect.
See T380409: Ensure quickstart provisions a working wikilambda.
Change 894584 had a related patch set uploaded (by Nik.xyz.in; author: Nik.xyz.in):
[mediawiki/extensions/WikiLambda@master] selenium-test: change assert to expect
Change 894584 merged by jenkins-bot:
[mediawiki/extensions/WikiLambda@master] selenium-test: change assert to expect
Change #1135095 had a related patch set uploaded (by Jforrester; author: Jforrester):
[mediawiki/extensions/WikiLambda@master] selenium tests: Fix one last use of assert
Change #1135095 merged by jenkins-bot:
[mediawiki/extensions/WikiLambda@master] selenium tests: Fix one last use of assert
As far as I can see, this is resolved. Please reopen if there's something left to do.