functional tests: update TestImageSize
functional tests: update TestImageSize

We're now rendering the image when we fetch it if we use overlay fs.
Take that into account in the test.

coreos/rkt,jonboulle/rkt,cab105/rkt,Wilybald/rkt,kinvolk/rkt,tmrts/rkt,nhlfr/rkt,sstabellini/rkt,lucasallan/rkt,Wilybald/rkt,cab105/rkt,Wilybald/rkt,sgotti/rkt,yifan-gu/rocket,squeed/rkt,intelsdi-x/rkt,achanda/rkt,Wilybald/rkt,squeed/rkt,yifan-gu/rocket,cab105/rkt,mike1729/rkt,sstabellini/rkt,matthaias/rkt,achanda/rkt,kinvolk/rkt,ybubnov/rkt,monstermunchkin/rkt,kinvolk/rkt,jjlakis/rkt,kinvolk/rkt,rkt/rkt,jakon89/rkt,jonboulle/rkt,sstabellini/rkt,jonboulle/rkt,coreos/rkt,shobhit85/rkt,joshix/rkt,alban/rkt,lucasallan/rkt,s-urbaniak/rkt,jakon89/rkt,s-urbaniak/rkt,nhlfr/rkt,nhlfr/rkt,achanda/rkt,yifan-gu/rkt,jjlakis/rkt,yifan-gu/rocket,monstermunchkin/rkt,sgotti/rkt,monstermunchkin/rkt,tmrts/rkt,mike1729/rkt,lucasallan/rkt,dgonyeo/rkt,ybubnov/rkt,joshix/rkt,euank/rkt,dgonyeo/rkt,Shashwatsh/rkt,akshaykarle/rkt,intelsdi-x/rkt,euank/rkt,matthaias/rkt,sgotti/rkt,Shashwatsh/rkt,shobhit85/rkt,jjlakis/rkt,shobhit85/rkt,rkt/rkt,joshix/rkt,alban/rkt,squeed/rkt,rkt/rkt,rkt/rkt,ybubnov/rkt,intelsdi-x/rkt,yifan-gu/rkt,mike1729/rkt,Shashwatsh/rkt,yifan-gu/rkt,s-urbaniak/rkt,joshix/rkt,coreos/rkt,lucasallan/rkt,euank/rkt,alban/rkt,tmrts/rkt,dgonyeo/rkt,akshaykarle/rkt,matthaias/rkt,jakon89/rkt,akshaykarle/rkt