Confirm we can move the attenuator

Following https://github.com/DiamondLightSource/crystallography-bluesky/issues/11 we should now be able to move the attenuator to various positions.

We should make sure that the plan only finishes when the move is complete

## Acceptance Criteria
- We can move the attenuator to various positions on the beamline

Start by reading the linked issue #11 and locating the attenuator movement plan in the repository. Check how movement completion is handled, then verify that the attenuator reaches several requested positions and that the plan does not finish before each move completes.

Written by the indexing model from the issue text.