worker/instancepoller: TestBatching less race-y
worker/instancepoller: TestBatching less race-y

The existing TestBatching test was very prone to
failure due to the nature of the test. The test
ensures that requests to Instances are batched
using a token bucket.

Whereas the existing test streamed requests, the
new implementation compresses the addition of
requests into the point where we service an
Instances call.

Fixes lp:1319652

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