chore(CMake): guard Lts/Tests add_subdirectory when BUILD_TESTING is ON to allow partial checkouts

Lts/CMakeLists.txt

@@ -27,7 +27,11 @@ foreach(dir IN LISTS LTS_SUBDIRS)
 endforeach()
 
 IF (BUILD_TESTING)
-  ADD_SUBDIRECTORY(Tests)
+  if (EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/Tests")
+    ADD_SUBDIRECTORY(Tests)
+  else()
+    message(STATUS "Skipping missing Tests directory under Lts (BUILD_TESTING is ON)")
+  endif()
 ENDIF()
 
 OPTION(BUILD_DOCS "Build documentation as html files from Lts/Documentation" OFF)