build: retry testfront to get around occasional segfault in travis.
build: retry testfront to get around occasional segfault in travis.

NiLuJe/koreader,poire-z/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader,Markismus/koreader,frankyifei/koreader,houqp/koreader,koreader/koreader,mwoz123/koreader,chihyang/koreader,Frenzie/koreader,Hzj-jie/koreader,NickSavage/koreader,lgeek/koreader,mihailim/koreader,apletnev/koreader,pazos/koreader,robert00s/koreader,poire-z/koreader