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

Previously all the stubs headers under mbed-stubs-headers,
so this PR move all cellular stubs headers under
cellular/tests/UNITTESTS/double directory and update CMake
to include headers into mbed-stubs-cellular library to make
cellular stubs to be self-contained

connectivity/cellular/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -26,6 +26,11 @@ target_compile_definitions(mbed-stubs-cellular
         MDMRXD=NC
 )
 
+target_include_directories(mbed-stubs-cellular
+    PUBLIC
+        .
+)
+
 target_sources(mbed-stubs-cellular
     PRIVATE
         AT_CellularContext_stub.cpp