test_vendor_lib: Add test channel command transport and handler

This upload adds the transport and handler functionality for the test
channel. Test channel commands will be sent by the test channel
program and processed by library objects in a manner analogous to how
packets are received and handled from the HCI. The actual
implementation for the 4 test channel commands included here will be
in the next upload.

Bug: 21586676
Change-Id: I9354baedcbd6e48b8920a3ebf1afa69eb89834d9