This used to work fine. It's now failing, as can be seen in https://integration.wikimedia.org/ci/job/mwext-Wikibase-qunit/5849/ for example.
155009:2015-01-04 22:54:11 integration-slave1004 build5847: [789d9dd0] /jenkins-mwext-Wikibase-qunit-5847/load.php?..&* ErrorException from line 366 of /mnt/jenkins-workspace/workspace/mwext-Wikibase-qunit/includes/debug/logger/legacy/Logger.php: PHP Warning: file_put_contents(mw-debug.log): failed to open stream: Permission denied
Over 10,000 hits for this error, and no mw-debug.log file was created in the artefacts showing that it consistently failed every time. Not just some of the time.