Expand assert_* functions to work with all signal types

[jsouter]

Fixes #707
Fixes #717

Expands OneOfEveryThingDevice into a StandardDetector so we can call read_configuration, and adds tests to tests/core/test_signal.py.

pytest.approx does not work on a whole dict when Tables are present, so now having to manually loop over each key in a dict[str, Reading], but now assert_reading, assert_value and assert_configuration work with arrays and table types.

[coretl]

#714 adds pytest assert rewriting, so you can ditch the custom error messages, and also an ApproxTable class used in MonitorQueue which could be used here instead of the is_same function

[kivel]

This might link to #710

[coretl]

This might link to #710

The ophyd_async.testing module (and ophyd_async.*.testing modules) are the only places that should use assert, as they are meant for tests, and we want pytest to do its assert rewriting, so must actually use an assert

[coretl]

One minor comment then we're good to go!

[coretl]

Feel free to merge this when you're ready