https://github.com/juju/juju/pull/5543 attempted to remove the recover() bandaid in pinger.ping, but provoked other test failures. These are most likely the same underlying cause; pingers being Stop'd, but not confirmed stopped before the TearDownTest runs.
panic: Session already closed
goroutine 3555 [running]:
panic(0x14b72a0, 0xc82068c020)
/usr/lib/go-1.6/src/runtime/panic.go:464 +0x3e6
gopkg.in/mgo%2ev2.(*Session).cluster(0xc8205faea0, 0xc82068c000)
/home/ubuntu/juju-core_2.0-beta9/src/gopkg.in/mgo.v2/session.go:1549 +0x75
gopkg.in/mgo%2ev2.copySession(0xc8205faea0, 0xc8204bfd01, 0x2)
/home/ubuntu/juju-core_2.0-beta9/src/gopkg.in/mgo.v2/session.go:519 +0x30
gopkg.in/mgo%2ev2.(*Session).Copy(0xc8205faea0, 0x1d46e40)
/home/ubuntu/juju-core_2.0-beta9/src/gopkg.in/mgo.v2/session.go:1515 +0x3b
github.com/juju/juju/state/presence.(*Pinger).ping(0xc820837540, 0x0, 0x0)
/home/ubuntu/juju-core_2.0-beta9/src/github.com/juju/juju/state/presence/presence.go:699 +0x1fc
github.com/juju/juju/state/presence.(*Pinger).loop(0xc820837540, 0x0, 0x0)
/home/ubuntu/juju-core_2.0-beta9/src/github.com/juju/juju/state/presence/presence.go:656 +0x172
github.com/juju/juju/state/presence.(*Pinger).Start.func1(0xc820837540)
/home/ubuntu/juju-core_2.0-beta9/src/github.com/juju/juju/state/presence/presence.go:555 +0x38
created by github.com/juju/juju/state/presence.(*Pinger).Start
/home/ubuntu/juju-core_2.0-beta9/src/github.com/juju/juju/state/presence/presence.go:565 +0x37d
FAIL github.com/juju/juju/apiserver/client 36.274s
