sandbox: add test of wdt_gpio driver

It seems that no other test has claimed gpio_a:7 yet, so use that.

The only small wrinkle is modifying the existing wdt test to use
uclass_get_device_by_driver() since we now have two UCLASS_WDT
instances in play, so it's a little more robust to fetch the device by
driver and not merely uclass+index.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>