payload/persistence intermittent failure

[Michael Foord]

When run on wily, with go 1.3 or 1.5 there's an intermittent test failure:

$ go test github.com/juju/juju/payload/persistence

----------------------------------------------------------------------
FAIL: env_test.go:80: envPersistenceSuite.TestListAllEmpty

env_test.go:98:
    s.Stub.CheckCallNames(c,
        "Machines",
        "MachineUnits",
        "MachineUnits",
        "newUnitPersistence",
        "ListAll",
        "newUnitPersistence",
        "ListAll",
    )
/home/michael/canonical/src/github.com/juju/testing/stub.go:211:
    return c.Check(funcNames, jc.DeepEquals, expected)
... obtained []string = []string{"Machines", "MachineUnits", "newUnitPersistence", "ListAll", "newUnitPersistence", "ListAll", "MachineUnits"}
... expected []string = []string{"Machines", "MachineUnits", "MachineUnits", "newUnitPersistence", "ListAll", "newUnitPersistence", "ListAll"}
... mismatch at [2]: unequal; obtained "newUnitPersistence"; expected "MachineUnits"

----------------------------------------------------------------------
FAIL: env_test.go:51: envPersistenceSuite.TestListAllOkay

env_test.go:77:
    s.Stub.CheckCallNames(c,
        "Machines",
        "MachineUnits",
        "MachineUnits",
        "newUnitPersistence",
        "ListAll",
        "newUnitPersistence",
        "ListAll",
        "MachineUnits",
        "newUnitPersistence",
        "ListAll",
    )
/home/michael/canonical/src/github.com/juju/testing/stub.go:211:
    return c.Check(funcNames, jc.DeepEquals, expected)
... obtained []string = []string{"Machines", "MachineUnits", "newUnitPersistence", "ListAll", "MachineUnits", "MachineUnits", "newUnitPersistence", "ListAll", "newUnitPersistence", "ListAll"}
... expected []string = []string{"Machines", "MachineUnits", "MachineUnits", "newUnitPersistence", "ListAll", "newUnitPersistence", "ListAll", "MachineUnits", "newUnitPersistence", "ListAll"}
... mismatch at [2]: unequal; obtained "newUnitPersistence"; expected "MachineUnits"

OOPS: 17 passed, 2 FAILED
--- FAIL: Test (0.00 seconds)
FAIL