Laborious (7)
(Other definitions for arduous that I've seen before include "Demanding great effort" , "Extremely taxing" , "Gruelling, strenuous" , "Steep - hard" , "Requiring hard work" .)
(Other definitions for testing that I've seen before include "Subjecting to 15 ac" , "of a difficult nature" , "Trying out" , "tiresome" , "stressful" .)