selftests: watchdog: avoid keepalive flood

Calling `watchdog-test [options] -p 0` results in flooding the kernel
with WDIOC_KEEPALIVE. Fix this by enforcing 1 second as minimal/default
keepalive/ping rate.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>