build: include CommonUnitTestsPlaceholder when present for migration smoke-tests

Lts/Common/CMakeLists.txt

@@ -33,3 +33,9 @@ ADD_SUBDIRECTORY(ProjectConfig)
 ADD_SUBDIRECTORY(TopicEditorBase) # TODO: rename / move ?
 #ADD_SUBDIRECTORY(LtsEventTimeline)
 #ADD_SUBDIRECTORY(LtsTranslator)
+
+# Placeholder test scaffolding for incremental migration
+if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/CommonUnitTestsPlaceholder/CMakeLists.txt")
+  add_subdirectory(CommonUnitTestsPlaceholder)
+endif()
+