state/watcher intermittent test failure

I've seen this once so far, and running the test several times didn't reproduce it, but I think it's worth having it in case it pops again.

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FAIL: watcher_test.go:285: WatcherSuite.TestDoubleUpdate

[LOG] 30.40085 JUJU:DEBUG state/watcher: reading most recent document to ignore past history...
[LOG] 30.40369 JUJU:DEBUG state/watcher: loading new events from changelog collection...
insert("test", "a") => revno 2
[LOG] 30.46012 JUJU:DEBUG state/watcher: got request: watcher.reqSync{done:(chan bool)(nil)}
update("test", "a") => revno 3
update("test", "a") => revno 4
[LOG] 30.46609 JUJU:DEBUG state/watcher: got request: watcher.reqWatch{key:watcher.watchKey{c:"test", id:"a"}, info:watcher.watchInfo{ch:(chan<- watcher.Change)(0xf8401a70f0), revno:3}}
[LOG] 30.46613 JUJU:DEBUG state/watcher: loading new events from changelog collection...
[LOG] 30.46650 JUJU:DEBUG state/watcher: got changelog document: bson.D{bson.DocElem{Name:"_id", Value:"Q/\x19\xa3\xe2\x01\xcec\x89\x00\x00\x03"}, bson.DocElem{Name:"test", Value:bson.D{bson.DocElem{Name:"d", Value:[]interface {}{"a"}}, bson.DocElem{Name:"r", Value:[]interface {}{4}}}}}
[LOG] 30.46660 JUJU:DEBUG state/watcher: got changelog document: bson.D{bson.DocElem{Name:"_id", Value:"Q/\x19\xa3\xe2\x01\xcec\x89\x00\x00\x02"}, bson.DocElem{Name:"test", Value:bson.D{bson.DocElem{Name:"d", Value:[]interface {}{"a"}}, bson.DocElem{Name:"r", Value:[]interface {}{3}}}}}
[LOG] 30.46667 JUJU:DEBUG state/watcher: got changelog document: bson.D{bson.DocElem{Name:"_id", Value:"Q/\x19\xa3\xe2\x01\xcec\x89\x00\x00\x01"}, bson.DocElem{Name:"test", Value:bson.D{bson.DocElem{Name:"d", Value:[]interface {}{"a"}}, bson.DocElem{Name:"r", Value:[]interface {}{2}}}}}
watcher_test.go:295:
    assertNoChange(c, s.ch)
watcher_test.go:82:
    c.Fatalf("watch reported %v, want nothing", got)
... Error: watch reported {test a 4}, want nothing

OOPS: 21 passed, 1 FAILED
--- FAIL: TestPackage (5.12 seconds)