api: Fix clock waits in TestLaterPingFails
api: Fix clock waits in TestLaterPingFails

This test was passing more by luck than anything.

Fixes https://bugs.launchpad.net/juju/+bug/1632105

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