This particular QUnit test failure has been causing builds to fail in MediaWiki core and other extensions.
FAILED TESTS (1/1): ve.ui.MWWikitextStringTransferHandler (Cite) ✖ convert Error: Pending AJAX requests: 1 (active: 1)
I'm unsure as to what code initiates the faulty/untracked Ajax request, but it's likely either the above test itself or a test shortly before it.
The debug log from the above Jenkins builds shows the last HTTP fetch to be for a url /null which seems like a bug, but might be unrelated.
00:11:12.519 03 09 2018 18:46:49.694:DEBUG [middleware:source-files]: Requesting /null / 00:11:12.520 03 09 2018 18:46:49.695:DEBUG [middleware:source-files]: Fetching /null 00:11:12.521 03 09 2018 18:46:49.696:DEBUG [proxy]: proxying request - /null to 127.0.0.1:9412