Add tests proving lock queue cleanup on timeout and hard-kill

Adds LockStepTimeoutQueueTest with 5 tests that verify the lock queue
is properly cleaned up when builds are timed out, aborted, or
hard-killed while waiting for a lockable resource:

- timeoutWhileWaitingForLockClearsQueue: timeout step wrapping lock
- abortWhileWaitingForLockByLabelClearsQueue: executor.interrupt()
- timeoutMiddleBuildInQueuePreservesOrder: FIFO order preserved
- hardKillWhileWaitingForLockClearsQueueViaIsValid: doKill() with
  multiple waiters — isValid() fallback removes stale queue entry
- hardKillOnlyWaiterDoesNotBlockFutureBuilds: doKill() sole waiter

All tests pass, confirming the lockable-resources plugin correctly
handles both the clean stop() path and the isValid() fallback path.

Fixes #773

Author: mPokornyETM