sandbox: add test of wdt-uclass' watchdog_reset()

Check that the watchdog_reset() implementation in wdt-uclass behaves
as expected:

- resets all activated watchdog devices
- leaves unactivated/stopped devices alone
- that the rate-limiting works, with a per-device threshold

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