client/gc: Replace GC integration test with unit
client/gc: Replace GC integration test with unit

The previous integration test was broken during the client refactor, and
it seems to be some sort of race with state updating.

I'm going to try and construct a replacement test as part of work on
performance, but for now, the underlying behaviour is still being
tested.

dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad