tests: add tcp connection test

The test creates and destroys threads, each of which creates a random number
of connections, each transferring a random number of bytes to an echo server.

This is used to stress the tcp/ip stack.

The test is portable, and builds on the host with the command

  g++ -O2  -g3 -pthread -std=gnu++11 -lboost_program_options -lboost_system tests/tst-tcp.cc

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>