Code review comment for lp:~nskaggs/juju-ci-tools/add-snap-to-default-juju-args

I attempted to reuse get_full_path for this, it can be nice, make it a utility function. However, it caused spiraling headaches all across the testsuite; into every single test. TestParseArgs everywhere needed a new mock to prevent the popen call. Given that, I think I'd prefer this.