Move mock aperture scatterguard logic into the mock device

[DominicOram]

Currently there is quite a bit of logic in tests/devices.conftest that is used to give a vaguely usable aperture_scatterguard device. Similar logic also exists in mx-bluesky. Following bluesky/ophyd-async#1039 we can now make this logic the default for the device and remove it from the tests of dodal/mx-bluesky.

Acceptance Criteria

• When you create a mock ApertureScatterguard you get something like the one created in the conftest
• The logic for this is no longer added in both the dodal and mx-bluesky tests