tests fff_fake_contexts: fix board name

Use full board name in cmake file.
Akin to the fix done in #80270
#80270
following the changes from
#77250
In which twister now uses the full board name when calling cmake.

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

tests/subsys/testsuite/fff_fake_contexts/CMakeLists.txt

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 cmake_minimum_required(VERSION 3.20.0)
-if(BOARD STREQUAL unit_testing)
+if(BOARD STREQUAL unit_testing/unit_testing)
   find_package(Zephyr COMPONENTS unittest REQUIRED HINTS $ENV{ZEPHYR_BASE})
   set(target testbinary)
 else()