apiserver/instancepoller: fix data race in test
apiserver/instancepoller: fix data race in test

Fixes LP 1461385

mhilton/juju,gabriel-samfira/juju,fwereade/juju,voidspace/juju,kat-co/juju,Altoros/juju-cloudsigma,perrito666/juju,mikemccracken/juju,wwitzel3/juju,macgreagoir/juju,reedobrien/juju,bz2/juju,frankban/juju,bac/juju,mikemccracken/juju,dooferlad/juju,marcmolla/juju,bogdanteleaga/juju,perrito666/juju,davecheney/juju,alesstimec/juju,frankban/juju-tmp,AdamIsrael/juju,alesstimec/juju,dimitern/juju,fwereade/juju,mjs/juju,makyo/juju,gabriel-samfira/juju,wwitzel3/juju,frankban/juju,bz2/juju,thomnico/juju,mikemccracken/juju,tych0/juju,kat-co/juju,mhilton/juju,makyo/juju,macgreagoir/juju,axw/juju,bac/juju,cloud-green/juju,mhilton/juju-juju,anastasiamac/juju,waigani/juju,bac/juju,howbazaar/juju,waigani/juju,macgreagoir/juju,mhilton/juju-juju,voidspace/juju,mhilton/juju-juju,axw/juju,fwereade/juju,thomnico/juju,bogdanteleaga/juju,davecheney/juju,anastasiamac/juju,mhilton/juju-juju,perrito666/juju,mjs/juju,tsakas/juju,mjs/juju,thomnico/juju,cloud-green/juju,cloud-green/juju,tasdomas/juju,mhilton/juju,waigani/juju,howbazaar/juju,kat-co/juju,waigani/juju,bz2/juju,axw/juju,makyo/juju,thomnico/juju,bz2/juju,bogdanteleaga/juju,Altoros/juju-cloudsigma,dooferlad/juju,Altoros/juju-cloudsigma,mjs/juju,mhilton/juju,alesstimec/juju,voidspace/juju,mikemccracken/juju,mjs/juju,AdamIsrael/juju,tsakas/juju,marcmolla/juju,tych0/juju,anastasiamac/juju,wwitzel3/juju,bac/juju,dooferlad/juju,bac/juju,dooferlad/juju,frankban/juju,makyo/juju,ericsnowcurrently/juju,axw/juju,frankban/juju-tmp,tasdomas/juju,frankban/juju,marcmolla/juju,marcmolla/juju,Altoros/juju-cloudsigma,anastasiamac/juju,tych0/juju,perrito666/juju,macgreagoir/juju,perrito666/juju,gabriel-samfira/juju,gabriel-samfira/juju,mikemccracken/juju,ericsnowcurrently/juju,mjs/juju,alesstimec/juju,reedobrien/juju,tych0/juju,tasdomas/juju,gabriel-samfira/juju,bz2/juju,mjs/juju,kat-co/juju,frankban/juju-tmp,axw/juju,tasdomas/juju,bogdanteleaga/juju,ericsnowcurrently/juju,alesstimec/juju,frankban/juju-tmp,reedobrien/juju,AdamIsrael/juju,davecheney/juju,dimitern/juju,cloud-green/juju,howbazaar/juju,reedobrien/juju,dimitern/juju,fwereade/juju,howbazaar/juju,anastasiamac/juju,dimitern/juju,frankban/juju,tsakas/juju,davecheney/juju,wwitzel3/juju,dooferlad/juju,tsakas/juju,ericsnowcurrently/juju,howbazaar/juju,dimitern/juju,macgreagoir/juju,marcmolla/juju,ericsnowcurrently/juju,reedobrien/juju,AdamIsrael/juju,kat-co/juju,voidspace/juju