Factor out cmd testing helpers.
Merge pull request #734 from ericsnowcurrently/cmd-testing-helpers

Factor out cmd testing helpers.

For the backups CLI I'm adding the subcommands in a subpackage of cmd/juju.  For that to work, I need some of the testing helpers in cmd/juju to live in their own package.  This patch makes that happen.  This is basically a copy-and-paste patch.