Make bgp_server_test more robust

Checking the BgpSessionManager queue size made the test fragile
because the sender could go into OpenSent state before receiver
has added the sesssion to it's WorkQueue.

Checking the hold timer expiry count is sufficient to ensure the
intent of the test is honored.

Change-Id: If6dd4a31b38c007c9dfd7ce9d76e023b5d6e94bb