CMake: Move netsocket stub headers to the netsocket doubles dir

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

connectivity/cellular/tests/UNITTESTS/framework/device/cellularcontext/CMakeLists.txt

@@ -40,6 +40,7 @@ target_link_libraries(${TEST_NAME}
         mbed-headers-cellular
         mbed-stubs
         mbed-stubs-cellular
+        mbed-stubs-netsocket
         mbed-stubs-headers
         gmock_main
 )

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

@@ -10,6 +10,11 @@ target_include_directories(mbed-headers-netsocket
 
 add_library(mbed-stubs-netsocket)
 
+target_include_directories(mbed-stubs-netsocket
+    PUBLIC
+        .
+)
+
 target_sources(mbed-stubs-netsocket
     PRIVATE
         CellularInterface_stub.cpp

connectivity/netsocket/tests/UNITTESTS/netsocket/CellularNonIPSocket/CMakeLists.txt

@@ -27,6 +27,7 @@ target_link_libraries(${TEST_NAME}
         mbed-headers-cellular
         mbed-stubs
         mbed-stubs-cellular
+        mbed-stubs-netsocket
         mbed-stubs-headers
         gmock_main
 )