need lockdir state machine tests for waiting, stealing, etc

[Martin Pool]

lockdir needs some tests for the behaviour of multiple interacting clients. It previously had some that used multiple threads, but they were disabled and then later taken out <https://code.launchpad.net/~mbp/bzr/cleanup/+merge/55465>.

It might be better to make a time- and side-effect-independent state machine, then test that in isolation.