Write docs on how to do testing

[coretl]

Need to document 2 ways:

1. Exercise the device directly, i.e. call device.read() and inspect the output. Should use helpers made in Test helpers for assering value, reading and configuration #199
2. Run a plan in the RunEngine and examine the docs. Use a pattern like:

from collections import defaultdict

def assert_emitted(docs: Dict[str, list], **numbers: int):
    assert list(docs) == list(numbers)
    assert {name: len(d) for name, d in docs.items()} == numbers

def test_plan():
    docs = defaultdict(list)
    RE(plan(), lambda name, doc: docs[name].append(doc))
    assert_emitted(docs, start=1, descriptor=1, resource=1, datum=1, event=1, stop=1)

[callumforrester]

There is an existing docs page on testing that we can add to: https://blueskyproject.io/ophyd-async/main/user/how-to/write-tests-for-devices.html