Fix an assert that wasn't testing anything.

[jameinel]

Description of change

In one of the recent refactorings, I pulled out a helper
function to test whether an entity was alive. I turned out
that the helper was broken in many ways.
It hard coded the 'key', but that didn't end up mattering because
it ended up doing:
c.Assert(alive, gc.Equals, alive)
which will always be true (it reused a parameter as a local variable).

The test happened to be correct anyway, this just fixes it to actually be
testing what we thought it was.

QA steps

Internal test change. You could change one of the values from "assertAlive" calls and see that it actually fails when it is supposed to instead of universally succeeding.

Documentation changes

No.

Bug reference

None (drive by while fixing a bug, but not directly related)

[jameinel]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju