[SYCL][Graph] Unitest bugfix (#308)

Returns a dummy handler of commandbuffer in Mock piextCommandBufferCreate function

sycl/unittests/helpers/PiMockPlugin.hpp

@@ -1272,7 +1272,7 @@ inline pi_result
 mock_piextCommandBufferCreate(pi_context context, pi_device device,
  const pi_ext_command_buffer_desc *desc,
  pi_ext_command_buffer *ret_command_buffer) {
-
+ *ret_command_buffer = createDummyHandle<pi_ext_command_buffer>();
  return PI_SUCCESS;
 }