test: use unix sockets for iproto connections

Enabled use_unix_sockets_iproto option to use unix sockets
iproto provides the new way to handle the problem with
'Address already in use' error. It lets test-run appoint
unix sockets for LISTEN environment variable values.

Before this change the TcpPortDispatcher was used to
eliminate the race condition when two workers trying to use
the same port: the idea was that each worker used its own
ports range. Really these ports could race with client ports
(from, say, net.box or replication), which typically didn't
use bind() and so bound to a random available port (despite
any dispatched ranges) and could produce 'Address already in
use' error.

Closes: #4008

(cherry picked from commit 60f84cbf)