Test helpers for assering value, reading and configuration

[callumforrester]

Dodal developers have produced various useful ways to test ophyd-async devices such as verify(). Some of these should probably make it back into main and sit alongside other helper functions like set_sim_signal.

I think we need 3 helper functions:

• assert_value(signal: SignalR[T], value: T)
• assert_reading(readable: Readable, reading: Reading)
• assert_configuration(readable: Readable, configuration: Configuration)

Acceptance Criteria

• Functions in ophyd-async
• Tests provided
• Docs provided where appropriate
• Issues raised in dodal to replace verefy() and any other old functions that can be superseded

[callumforrester]

@coretl

[coretl]

Possibly also want assert_emitted from #206