cubicweb #4294727 Fix QUnitTestCase [validation pending]

Broken again…

prioritynormal
typebug
done in3.19.5
load left0.000
closed by#3905682ece11 [devtools] Fix Firefox launcher in QUnitTestCase (closes #4294727)
patch[devtools] Fix Firefox launcher in QUnitTestCase (closes #4294727) [applied]