CMake: Move events stub headers to the events/tests/UNITTESTS/double dir

Move events stubs headers to the mbed-os/events/tests/UNITTESTS/double
directory and update double CMake to include stub headers

Author: rajkan01

events/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -23,6 +23,11 @@ target_sources(mbed-stubs-events
         mbed_shared_queues_stub.cpp
 )
 
+target_include_directories(mbed-stubs-events
+    PUBLIC
+        .
+)
+
 target_link_libraries(mbed-stubs-events
     PRIVATE
         mbed-headers