Fix release build by guarding add_subdirectory(test)

The release workflow removes the test/ directory from the archive
before running the build command. The unconditional
add_subdirectory(test) in CMakeLists.txt causes CMake to fail with:

  add_subdirectory given source "test" which is not an existing directory.

Guard it with IS_DIRECTORY so the build succeeds when test/ is absent.

Author: archigup

CMakeLists.txt

@@ -171,7 +171,9 @@ FetchContent_Declare( cmock
 
 add_subdirectory(source)
 add_subdirectory(tools)
-add_subdirectory(test)
+if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/test")
+    add_subdirectory(test)
+endif()
 
 if(STANDALONE_TEST_BUILD_UNIX)
     add_library(freertos_config INTERFACE)