docs: add test for firmware.json QAPI

docs: add test for firmware.json QAPI

To make sure that the QAPI description stays valid, add a testcase.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/d9ce0234-4beb-4b90-b14c-76810d3b81d7@linaro.org/
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240724-qapi-firmware-json-v7-1-12341f7e362d@linutronix.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>