tests: invoke gofmt with simplify-code flag
tests: invoke gofmt with simplify-code flag

Enables code simplification checks of gofmt.

Related to #2462

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