When playing with the job testing multiple extensions together ( https://www.mediawiki.org/wiki/RFC/Extensions_continuous_integration ), I found out the Flow tests are failing because they can not contact Parsoid although the PHPUnit job for Flow pass just fine.
Talking with @matthiasmullie , the reason is that the shared job bring in VisualEditor and Flow will fallback its Parsoid config ( $wgFlowParsoid* ) to whatever VisualEditor has ( $wgVisualEditorParsoidURL = 'http://localhost:8000'; ).
Since we do not have yet a Parsoid backend spawned in the job, Flow fails miserably. A way to fix it would be to disable Parsoid configuration entirely whenever it detects its run under Jenkins. We have a global variable set for that: $wgWikimediaJenkinsCI = true; which has been used reliably by the Wikibase repositories.