cmd/jujud: fix diskmanager tests on Windows

- Add test that checks worker is started
- Change tests that waits for state to be
  updated to only run on Linux.
worker/diskmanager: test NewWorker

- Test NewWorker runs a no-op worker on Windows.
- Test NewWorker eventually calls the BlockDeviceSetter
  on Linux.
- Shuffle code around so it works on Linux and Windows.