tests: fix abuses of appc types.Isolator
tests: fix abuses of appc types.Isolator

appc 0.8.5 perform stricter validation on types.Isolator instances,
and some rkt tests would fail due to library misuses.
This commit switches to proper unmarshaling to construct those
instances.

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