tests: wdt_basic_api: make mec15xxevb_assy6853 build only

The bootloader on mec15xxevb_assy6853 clears the memory on reboot,
which would fail the wdt_basic_api test. Instead of simply exclude
the platform, add a build only test for this board so the watchdog
is being built to avoid build breakage.

Fixes #20301

Signed-off-by: Daniel Leung <daniel.leung@intel.com>