samples: tests: don't disable watchdog when it is used

To ease maintenance of samples and tests some SoCs define
CONFIG_WDT_DISABLE_AT_BOOT=y to disable the watchdog. Ensure the option
is set to n for samples and tests that require watchdog module not to be
disabled during boot.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>