Factor common code out of fake destinations

tests/fakes/dest/hello_wrong_size.rb

@@ -4,40 +4,20 @@
 # a valid NBD hello with a random size, then check that we have see an
 # EOF on read.
 
-addr, port = *ARGV
+require 'flexnbd/fake_dest'
+include FlexNBD::FakeDest
 
-require 'socket'
-require 'timeout'
-require 'flexnbd/constants'
+sock = serve( *ARGV )
+client_sock = accept( sock, "Timed out waiting for a connection" )
 
-sock = TCPServer.new( addr, port )
-client_sock = nil
-
-begin
-  Timeout.timeout(2) do
-    client_sock = sock.accept
-  end
-rescue Timeout::Error
-  $stderr.puts "Timed out waiting for a connection"
-  exit 1
-end
 
 t = Thread.new do
-  begin
-    Timeout.timeout(FlexNBD::MS_RETRY_DELAY_SECS + 1) do
-      client_sock2 = sock.accept
-    end
-  rescue Timeout::Error
-    $stderr.puts "Timed out waiting for a reconnection"
-    exit 1
-  end
+  client_sock2 = accept( sock, "Timed out waiting for a reconnection",
+                         FlexNBD::MS_RETRY_DELAY_SECS + 1 )
+  client_sock2.close
 end
 
-client_sock.write( "NBDMAGIC" )
-client_sock.write( "\x00\x00\x42\x02\x81\x86\x12\x53" )
-8.times do client_sock.write rand(256).chr end
-client_sock.write( "\x00" * 128 )
-
+write_hello( client_sock, :size => :wrong )
 
 t.join