CMake: Move events fakes into events double dir

Previously events fakes as part of UNITTESTS/fakes, this PR moves
events fakes to events double directory to make events stubs
to be self-contained.

UNITTESTS/fakes/CMakeLists.txt

@@ -1,5 +1,4 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-add_subdirectory(events)
 add_subdirectory(ble)

events/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -1,6 +1,8 @@
 # Copyright (c) 2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
+add_subdirectory(fakes)
+
 add_library(mbed-headers-events INTERFACE)
 
 target_include_directories(mbed-headers-events