Factor common code out of fake destinations

tests/fakes/dest/reject_acl.rb

@@ -2,20 +2,11 @@
 
 # Accept a connection, then immediately close it.  This simulates an ACL rejection.
 
-addr, port = *ARGV
-require 'socket'
-require 'timeout'
+require 'flexnbd/fake_dest'
+include FlexNBD::FakeDest
 
-serve_sock = TCPServer.open( addr, port )
-
-begin
-  Timeout.timeout( 2 ) do
-    serve_sock.accept.close
-  end
-rescue Timeout::Error
-  $stderr.puts "Timed out waiting for a connection"
-  exit 1
-end
+serve_sock = serve( *ARGV )
+accept( serve_sock, "Timed out waiting for a connection" ).close
 
 serve_sock.close