functional test: TestExitCode: fix test
functional test: TestExitCode: fix test

Since commit 76ed159 ("functional tests: TestFailure: don't rely on
systemd debug logs"), TestExitCode didn't test anything and always
passed.

Fortunately, there was no regression bugs introduced in rkt. So no harm
done. But let's fix the test to catch potential future regression bugs.

sinfomicien/rkt,ybubnov/rkt,il9ue/rkt,alban/rkt,s-urbaniak/rkt,maquanyi/rkt,akshaykarle/rkt,yifan-gu/rkt,il9ue/rkt,jzelinskie/rkt,crawford/rkt,cab105/rkt,euank/rkt,bsmr-go/rkt,dalanlan/rkt,jonboulle/rkt,yifan-gu/rocket,marineam/rkt,sgotti/rkt,krnowak/rkt,luebken/rkt,lucasallan/rkt,markllama/rkt,coreos/rkt,joshix/rkt,jasonchaffee/rkt,tixxdz/rkt,dgonyeo/rkt,Yhgenomics/rkt,liugenping/rkt,iankronquist/rkt,mattthias/pkg-rkt,monstermunchkin/rkt,akshaykarle/rkt,Wilybald/rkt,jzelinskie/rkt,tv42/rkt,bsmr-go/rkt,rkt/rkt,cab105/rkt,sgotti/rkt,wparad/rkt,lee212/rkt,tixxdz/rkt,lee212/rkt,il9ue/rkt,mike1729/rkt,dalanlan/rkt,wparad/rkt,rot26/rkt,chrisseto/rkt,lucasallan/rkt,sgotti/rkt,alban/rkt,rhvgoyal/rkt,blixtra/rkt,crawford/rkt,blixtra/rkt,squaremo/rkt,liubq919/rkt,dgonyeo/rkt,matthaias/rkt,yifan-gu/rkt,shtouff/rkt,adfernandes/rkt,MattDees/rkt,michelangelo13/rkt,crawford/rkt,kbrwn/rkt,iankronquist/rkt,euank/rkt,linki/rkt,achanda/rkt,marineam/rkt,iaguis/rkt,tixxdz/rkt,intfrr/rkt,chrisseto/rkt,liubq919/rkt,jjlakis/rkt,yifan-gu/rkt,endocode/rkt,joshix/rkt,rhvgoyal/rkt,yifan-gu/rocket,endocode/rkt,intelsdi-x/rkt,rot26/rkt,rkt/rkt,fdserr/rkt,lucasallan/rkt,sinfomicien/rkt,jasonchaffee/rkt,squeed/rkt,sinfomicien/rkt,maquanyi/rkt,fdserr/rkt,wparad/rkt,rkt/rkt,Shashwatsh/rkt,jakon89/rkt,liugenping/rkt,luebken/rkt,jjlakis/rkt,joshix/rkt,dgonyeo/rkt,kinvolk/rkt,tv42/rkt,alexhersh/rkt,euank/rkt,ChengTiesheng/rkt,ChengTiesheng/rkt,lucasallan/rkt,iaguis/rkt,iankronquist/rkt,shobhit85/rkt,linki/rkt,krnowak/rkt,Yhgenomics/rkt,sstabellini/rkt,sstabellini/rkt,coreos/rkt,s-urbaniak/rkt,bsmr-go/rkt,mike1729/rkt,michelangelo13/rkt,MattDees/rkt,Yhgenomics/rkt,cab105/rkt,jjlakis/rkt,jakon89/rkt,alexhersh/rkt,marineam/rkt,markllama/rkt,iaguis/rkt,kbrwn/rkt,squaremo/rkt,tmrts/rkt,mjg59/rkt,squeed/rkt,adfernandes/rkt,nhlfr/rkt,endocode/rkt,achanda/rkt,nhlfr/rkt,liubq919/rkt,markllama/rkt,joshix/rkt,smarvin/rkt,smarvin/rkt,matthaias/rkt,squaremo/rkt,achanda/rkt,jonboulle/rkt,Shashwatsh/rkt,tmrts/rkt,mjg59/rkt,tv42/rkt,alban/rkt,monstermunchkin/rkt,Wilybald/rkt,kinvolk/rkt,Wilybald/rkt,Shashwatsh/rkt,monstermunchkin/rkt,adfernandes/rkt,mjg59/rkt,jakon89/rkt,shtouff/rkt,lee212/rkt,Wilybald/rkt,rkt/rkt,akshaykarle/rkt,ybubnov/rkt,rhvgoyal/rkt,shobhit85/rkt,maquanyi/rkt,intfrr/rkt,jonboulle/rkt,kinvolk/rkt,intfrr/rkt,kbrwn/rkt,mattthias/pkg-rkt,rot26/rkt,nhlfr/rkt,kinvolk/rkt,sstabellini/rkt,ybubnov/rkt,ChengTiesheng/rkt,michelangelo13/rkt,squeed/rkt,smarvin/rkt,blixtra/rkt,jasonchaffee/rkt,mattthias/pkg-rkt,luebken/rkt,shtouff/rkt,alexhersh/rkt,fdserr/rkt,intelsdi-x/rkt,jzelinskie/rkt,mike1729/rkt,liugenping/rkt,coreos/rkt,dalanlan/rkt,krnowak/rkt,MattDees/rkt,yifan-gu/rocket,matthaias/rkt,intelsdi-x/rkt,linki/rkt,chrisseto/rkt,s-urbaniak/rkt,shobhit85/rkt,tmrts/rkt