tests: watchdog: Update test to work with the mcux watchdog driver

Makes the dual timeout test conditional upon the nrfx watchdog driver
because not all watchdogs can support multiple timeouts.

Test now passes on frdm_k64f, hexiwear_k64, and usb_kw24d512 boards.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>