Add test context support for memorized attributes

[DrewDevereux]

This pull request provides DeviceTestContext and MultiDeviceTestContext support for memorized attributes. This has value because it allows lightweight testing of device initialisation where device attributes are memorized.

The implementation provides a memorized argument that is handled just like the properties argument:

• For DeviceTestContext, a new constructor argument is added:

  with DeviceTestContext(TestDevice, memorized={"attr": "1"}) as proxy:
   asserty proxy.attr == 1

• For MultiDeviceTestContext, the devices_info structure has a new memorized key:

  devices_info = (
    {
        "class": TestDevice,
        "devices": [
            {
                "name": "test/device1/1",
                "memorized": {"attr": "1"}
            },
        ]
    }
  )
  with MultiDeviceTestContext(devices_info) as context:
    proxy = context.get_device("test/device1/1")
    assert proxy.attr == 1