apiserver/client: refactor client tests

Updates LP #1386968
Make it crystal clear that s.makeMockCharmStore overwrites client.CharmStore.
use baseSuite.AddCleanup, thanks menn0
Merge pull request #986 from davecheney/206-apiserver-client-refactor-tests

apiserver/client: refactor client tests

Make it crystal clear that s.makeMockCharmStore overwrites client.CharmStore

Updates LP #1386968