* statemgr: add a NewUnlockErrorFull state manager for tests
I've frequently needed to coerce Unlock() errors for tests and it's been
awkward and fraught every time, so I decided to add a full state manger
that returns *mostly* errors. I intend to use this in conjunction with
the clistate.Locker interface, which first calls Lock() (to block if the
mutex is in use) at the start of Unlock(), so Lock() rather awkwardly needed to succeed.
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