Panic session closed still happening in unit tests

Recent ci unit test jobs have been consistently failing with "panic: Session already closed".

Revision c3bb7a6e0d43bcbba68cf2a480d3b3ae830730f4 was trying to fix one failure point.
<http://github.com/juju/juju/pull/1499>
<http://reviews.vapour.ws/r/821/>

However it seems we are still getting the panic:

<http://reports.vapour.ws/releases/2282>

<http://data.vapour.ws/juju-ci/products/version-2282/run-unit-tests-precise-i386/build-1323/consoleText>
In cmd/jujud/agent/machine_test.go:
ok github.com/juju/juju/cmd/jujud 32.778s
panic: Session already closed
...
goroutine 4990 [chan receive]:
launchpad.net/tomb.(*Tomb).Wait(0x1a104e48, 0x0, 0x0)
 /home/ubuntu/juju-core_1.23-alpha1/src/launchpad.net/tomb/tomb.go:110 +0x42
github.com/juju/juju/state/presence.(*Pinger).Stop(0x1a104e40, 0x0, 0x0)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/state/presence/presence.go:528 +0x171
github.com/juju/juju/cmd/jujud/agent.func·074(0x19e36640)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/cmd/jujud/agent/machine_test.go:170 +0x2c
github.com/juju/testing.(*CleanupSuite).callStack(0x19bac390, 0x19e36640, 0x19d1dbc0, 0x7, 0x8)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/testing/cleanup.go:41 +0x4b
github.com/juju/testing.(*CleanupSuite).TearDownTest(0x19bac390, 0x19e36640)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/testing/cleanup.go:36 +0x40
github.com/juju/testing.(*FakeHomeSuite).TearDownTest(0x19bac390, 0x19e36640)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/testing/home.go:114 +0x3e
github.com/juju/juju/testing.(*FakeJujuHomeSuite).TearDownTest(0x19bac384, 0x19e36640)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/testing/environ.go:151 +0x54
github.com/juju/juju/cmd/jujud/agent.(*commonMachineSuite).TearDownTest(0x19bac380, 0x19e36640)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/cmd/jujud/agent/machine_test.go:153 +0x3d
...

? github.com/juju/juju/feature [no test files]
panic: Session already closed

goroutine 851 [semacquire]:
sync.runtime_Syncsemacquire(0x19dc19a0)
 /usr/lib/go/src/pkg/runtime/sema.goc:257 +0xd0
sync.(*Cond).Wait(0x19dc1998)
 /usr/lib/go/src/pkg/sync/cond.go:62 +0x7a
gopkg.in/mgo%2ev2.(*mongoCluster).AcquireSocket(0x19dc1980, 0x19dc1901, 0xf8475800, 0xd, 0xf8475800, ...)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/cluster.go:546 +0x65c
gopkg.in/mgo%2ev2.(*Session).acquireSocket(0x19d70750, 0x19c57501, 0x0, 0x0, 0x0)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:3408 +0x243
gopkg.in/mgo%2ev2.(*Query).One(0x19dc1900, 0x0, 0x0, 0x0, 0x0)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:2365 +0xa1
gopkg.in/mgo%2ev2.(*Database).Run(0x19cd1820, 0x88487a0, 0x19cd1840, 0x0, 0x0, ...)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:580 +0x1a1
gopkg.in/mgo%2ev2.(*Session).Run(0x19d70750, 0x8723820, 0x19f011b0, 0x0, 0x0, ...)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:1639 +0x9f
gopkg.in/mgo%2ev2.(*Session).Ping(0x19d70750, 0x19dc1980, 0xf8475800)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:1668 +0x59
gopkg.in/mgo%2ev2.DialWithInfo(0x19c57480, 0x19c57480, 0x8b049b4, 0x40996c04)
 /home/ubuntu/juju-core_1.23-alpha1/src/gopkg.in/mgo.v2/session.go:391 +0x3df
github.com/juju/testing.(*MgoInstance).MustDial(0x9762460, 0x19f3c060)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/testing/mgo.go:414 +0x37
github.com/juju/testing.(*MgoInstance).Reset(0x9762460)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/testing/mgo.go:500 +0xad
github.com/juju/juju/provider/dummy.(*environState).destroy(0x19cf6960)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/provider/dummy/environs.go:332 +0x143
github.com/juju/juju/provider/dummy.Reset()
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/provider/dummy/environs.go:307 +0x100
github.com/juju/juju/juju/testing.(*JujuConnSuite).tearDownConn(0x19a3ea88, 0x19d66e60)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/juju/testing/conn.go:507 +0x426
github.com/juju/juju/juju/testing.(*JujuConnSuite).TearDownTest(0x19a3ea88, 0x19d66e60)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/juju/testing/conn.go:109 +0x33
github.com/juju/juju/featuretests.(*leadershipSuite).TearDownTest(0x19a3ea80, 0x19d66e60)
 /home/ubuntu/juju-core_1.23-alpha1/src/github.com/juju/juju/featuretests/leadership_test.go:111 +0x116
...