test_vendor_lib: Add test channel for run-time input

This change adds a test channel for receiving user-specified commands
and data at run-time. Specifically, the channel is intended to be used
for additional debugging capabilities and for fine-tuned control over
the test controller. Implementation-wise, the test channel is simply
another socket that the vendor manager watches on. The handling of test
channel input goes through the pipeline as normal HCI data after it is
received by the HciTransport.

A script for building and running the test vendor library and
(optionally) the test channel has been added in scripts/.

Bug: 21586676
Change-Id: I55bdeedbcd81effbe009aa62a19031637374a1e6