test_vendor_lib: Use EPOLLOUT

HciTransport checks for write-readiness before sending any packets
back to the HCI. Packets are maintained on a queue and only written
when the EPOLLOUT flag is set in the epoll_event.

Bug: 21586676
Change-Id: Iebd9599286b62746e2be7e424c522058ae978bb1
10 files changed