Compatibility with ophyd async 0.3a4

[olliesilvester]

Changes we need to be aware of for some of our Hyperion tests:

• We now have soft_signal_r_and_setter in ophyd async which returns the signal and its setter. This replaces our old way of using soft signals and gives us a way to change the values without using the _backend
• Sim signals are now called mock signals, along with similar changes in some of their method names