unitUpgraderSuite.SetUpTest cannot add unit to service

[Curtis Hovey]

As seen at
    http://reports.vapour.ws/releases/issue/57bfe296749a566cb0189761

unitupgrader_test.go:48:
    s.rawMachine, _, _, s.rawUnit = s.addMachineServiceCharmAndUnit(c, "wordpress")
unitupgrader_test.go:66:
    c.Assert(err, jc.ErrorIsNil)
... value *errors.Err = &errors.Err{message:"cannot add unit to service \"wordpress\"", cause:(*errors.errorString)(0xc20800a0a0), previous:(*errors.errorString)(0xc20800a0a0), file:"github.com/juju/juju/state/service.go", line:818} ("cannot add unit to service \"wordpress\": EOF")

[Anastasia (anastasia-macmood)]

This must have been fixed as part of some work since I cannot no longer reproduce locally under stress.