CMake: Fix DTLS socket unittest

- Binding the socket address into network stack to avoid socket connect API call hangs in the unittest

Author: rajkan01

connectivity/netsocket/tests/UNITTESTS/netsocket/DTLSSocket/test_DTLSSocket.cpp

@@ -62,5 +62,6 @@ TEST_F(TestDTLSSocket, connect)
 
     stack.return_value = NSAPI_ERROR_OK;
     SocketAddress a("127.0.0.1", 1024);
+    stack.return_socketAddress = a;
     EXPECT_EQ(socket->connect(a), NSAPI_ERROR_OK);
 }