[merge][#109520] gc of LockableFiles should not warn or unlock

[Martin Pool]

>  Well, I feel like sweeping it under the rug is hiding something which is broken
>  in our test suite.

It seems like what we really want is to check at the end of each test
whether any locks are still held.  I can think of two ways to do that
check from the end in the test case cleanup:

 * scan the transport looking for any held locks (trivially, just for
any directories called "held" perhaps)
 * keep in a global variable a list of all the locks we currently have
held, and assert that it must be zero

Both of these are better in that they will cause a failure in the
actual test where the problem occurred, and because they can't so
easily be ignored.

-- 
Martin <http://launchpad.net/~mbp/>