state/apiserver/testing: FakeAuthorizer fixups

This PR removes three of the boolean fields on FakeAuthorizer. There is more work to be done, but this is a good point to land what I have done so far.

Discussion: https://lists.ubuntu.com/archives/juju-dev/2014-July/003104.html
None, apart from making it very clear that AuthMachineAgent is derrived from GetAuthTag.

William has asked me to unify (not just copy pasta) this logic between the two authorizers, which I am doing in a followup.