UnitSuite.TestWatchUnit random failure

[Martin Packman]

Bot failed merge with an unexpected watcher change on TestWatchUnit in state tests.

FAIL: unit_test.go:1019: UnitSuite.TestWatchUnit

[LOG] 16.06797 INFO juju state: opening state; mongo addresses: ["localhost:38293"]; entity ""
 [LOG] 16.07628 INFO juju state: connection established
 [LOG] 16.10150 INFO juju state: initializing environment
 unit_test.go:1053:
     testing.NewNotifyWatcherC(c, s.State, w).AssertOneChange()
 testing/watcher.go:85:
     c.Fatalf("watcher sent unexpected change: (_, %v)", ok)
 ... Error: watcher sent unexpected change: (_, true)

OOPS: 307 passed, 1 FAILED
 --- FAIL: TestPackage (69.50 seconds)
 FAIL
 FAIL launchpad.net/juju-core/state 69.726s

<https://code.launchpad.net/~gz/juju-core/machine_doc_addresses/+merge/177978/comments/404003>

This is much the same as some other errors seen in the past, rog has a mitigation branch up for review:

<https://codereview.appspot.com/12352044/>