api/unitassigner: fix data race in test
api/unitassigner: fix data race in test

Fixes LP 1516498

mjs/juju,davecheney/juju,voidspace/juju,axw/juju,bogdanteleaga/juju,wwitzel3/juju,bac/juju,anastasiamac/juju,kat-co/juju,howbazaar/juju,howbazaar/juju,bz2/juju,perrito666/juju,anastasiamac/juju,bogdanteleaga/juju,fwereade/juju,fwereade/juju,dimitern/juju,perrito666/juju,waigani/juju,bogdanteleaga/juju,axw/juju,alesstimec/juju,alesstimec/juju,waigani/juju,dimitern/juju,bac/juju,dimitern/juju,kat-co/juju,dooferlad/juju,makyo/juju,axw/juju,bogdanteleaga/juju,frankban/juju,kat-co/juju,voidspace/juju,mikemccracken/juju,macgreagoir/juju,voidspace/juju,marcmolla/juju,AdamIsrael/juju,ericsnowcurrently/juju,wwitzel3/juju,mjs/juju,frankban/juju,voidspace/juju,marcmolla/juju,perrito666/juju,ericsnowcurrently/juju,perrito666/juju,mjs/juju,howbazaar/juju,macgreagoir/juju,mjs/juju,marcmolla/juju,AdamIsrael/juju,frankban/juju,makyo/juju,wwitzel3/juju,anastasiamac/juju,alesstimec/juju,bz2/juju,howbazaar/juju,AdamIsrael/juju,anastasiamac/juju,waigani/juju,davecheney/juju,kat-co/juju,axw/juju,gabriel-samfira/juju,mikemccracken/juju,gabriel-samfira/juju,makyo/juju,anastasiamac/juju,marcmolla/juju,frankban/juju,mikemccracken/juju,dooferlad/juju,waigani/juju,mjs/juju,frankban/juju,marcmolla/juju,dooferlad/juju,mikemccracken/juju,mjs/juju,reedobrien/juju,dimitern/juju,davecheney/juju,reedobrien/juju,davecheney/juju,dooferlad/juju,dimitern/juju,ericsnowcurrently/juju,mjs/juju,alesstimec/juju,bz2/juju,makyo/juju,reedobrien/juju,gabriel-samfira/juju,kat-co/juju,bac/juju,bz2/juju,bac/juju,ericsnowcurrently/juju,wwitzel3/juju,howbazaar/juju,fwereade/juju,gabriel-samfira/juju,axw/juju,AdamIsrael/juju,perrito666/juju,ericsnowcurrently/juju,fwereade/juju,reedobrien/juju,mikemccracken/juju,alesstimec/juju,macgreagoir/juju,gabriel-samfira/juju,bac/juju,macgreagoir/juju,reedobrien/juju,dooferlad/juju,bz2/juju,macgreagoir/juju