api: waitThenAdvance helper
api: waitThenAdvance helper

Reduces test boilerplate.

perrito666/juju,macgreagoir/juju,anastasiamac/juju,dimitern/juju,ericsnowcurrently/juju,macgreagoir/juju,dimitern/juju,AdamIsrael/juju,bac/juju,gabriel-samfira/juju,macgreagoir/juju,howbazaar/juju,frankban/juju,AdamIsrael/juju,anastasiamac/juju,reedobrien/juju,perrito666/juju,anastasiamac/juju,perrito666/juju,marcmolla/juju,waigani/juju,alesstimec/juju,waigani/juju,bac/juju,frankban/juju,AdamIsrael/juju,gabriel-samfira/juju,dimitern/juju,macgreagoir/juju,howbazaar/juju,anastasiamac/juju,ericsnowcurrently/juju,mjs/juju,perrito666/juju,mikemccracken/juju,frankban/juju,ericsnowcurrently/juju,gabriel-samfira/juju,bac/juju,marcmolla/juju,reedobrien/juju,waigani/juju,mjs/juju,reedobrien/juju,ericsnowcurrently/juju,bac/juju,bac/juju,marcmolla/juju,alesstimec/juju,mjs/juju,mikemccracken/juju,ericsnowcurrently/juju,mjs/juju,mikemccracken/juju,frankban/juju,howbazaar/juju,reedobrien/juju,macgreagoir/juju,mjs/juju,alesstimec/juju,marcmolla/juju,dimitern/juju,frankban/juju,gabriel-samfira/juju,mikemccracken/juju,gabriel-samfira/juju,anastasiamac/juju,AdamIsrael/juju,mjs/juju,mikemccracken/juju,howbazaar/juju,perrito666/juju,alesstimec/juju,howbazaar/juju,alesstimec/juju,reedobrien/juju,mjs/juju,waigani/juju,dimitern/juju,marcmolla/juju