Fix test build: use PRIVATE include scope and find Unity headers correctly

The test CMakeLists used INTERFACE instead of PRIVATE for
target_include_directories, which meant the Unity include path was
not applied to test_runner itself. Also replaced the incorrect
${UNITY} variable (a library path from find_library) with a proper
find_path call that locates unity.h under the unity/ subdirectory
where Unity's cmake install places it.

Author: papawattu

test/CMakeLists.txt

@@ -10,6 +10,7 @@ target_link_libraries (test_runner LINK_PUBLIC
     ${CJSON}
     unity
 )
-target_include_directories(test_runner INTERFACE ${UNITY})
+find_path(UNITY_INCLUDE_DIR unity.h PATH_SUFFIXES unity)
+target_include_directories(test_runner PRIVATE ${UNITY_INCLUDE_DIR})
 
 add_test(test_phev_core test_runner)