juju/service: test fixture panics if SetUpSuite fails

FAIL: upgradecharm_test.go:35: UpgradeCharmErrorsSuite.SetUpTest

upgradecharm_test.go:36:
    s.RepoSuite.SetUpTest(c)
/home/dfc/src/github.com/juju/testing/mgo.go:489:
    c.Assert(err, jc.ErrorIsNil)
... value *net.OpError = &net.OpError{Op:"local error", Net:"", Source:net.Addr(nil), Addr:net.Addr(nil), Err:0xa} ("local error: unexpected message")

----------------------------------------------------------------------
PANIC: upgradecharm_test.go:55: UpgradeCharmErrorsSuite.TearDownTest

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x45E29E)

/home/dfc/go/src/runtime/asm_amd64.s:472
  in call32
/home/dfc/go/src/runtime/panic.go:426
  in gopanic
/home/dfc/go/src/runtime/panic.go:45
  in panicmem
/home/dfc/go/src/runtime/sigpanic_unix.go:24
  in sigpanic
/home/dfc/src/gopkg.in/mgo.v2/session.go:1537
  in Session.Close
/home/dfc/src/github.com/juju/testing/mgo.go:611
  in MgoSuite.TearDownTest
/home/dfc/src/github.com/juju/juju/juju/testing/conn.go:122
  in JujuConnSuite.TearDownTest
/home/dfc/src/github.com/juju/juju/juju/testing/repo.go:80
  in RepoSuite.TearDownTest
upgradecharm_test.go:58
  in UpgradeCharmErrorsSuite.TearDownTest
/home/dfc/go/src/runtime/asm_amd64.s:472
  in call32
/home/dfc/go/src/reflect/value.go:303
  in Value.Call
/home/dfc/go/src/runtime/asm_amd64.s:1998
  in goexit

----------------------------------------------------------------------
PANIC: upgradecharm_test.go:117: UpgradeCharmErrorsSuite.TestPathAndRevisionFails

... Panic: Fixture has panicked (see related PANIC)
OOPS: 145 passed, 1 FAILED, 1 FIXTURE-PANICKED, 4 MISSED
--- FAIL: TestPackage (72.31s)
FAIL
FAIL github.com/juju/juju/cmd/juju/service 72.376s