tests/ztest/fail: fix board name

Use full board name in cmake file.
Akin to the fix done in
https://github.com/zephyrproject-rtos/zephyr/pull/80270/
following the changes from
https://github.com/zephyrproject-rtos/zephyr/pull/77250/



Note after the Zephyr cmake code has been run the BOARD
variable is split into BOARD BOARD_QUALIFIERS,
where BOARD does not contain the qualifiers anymore
(see cmake/modules/boards.cmake for more info).

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>