Issue #1357033: make a backups test more deterministic.
Merge pull request #525 from ericsnowcurrently/fix-1357033-broken-backups-test

Issue #1357033: make a backups test more deterministic.

Here is a solution to the failing test from [issue #1357033](https://bugs.launchpad.net/juju-core/+bug/1357033).  Incidentally, I took this basically verbatim from one of my outstanding PRs.