Make test sockets less dependent on enviroment

It seems that ruby in a default wheezy VM can't handle a source address
of nil.

tests/acceptance/flexnbd/fake_source.rb

@@ -9,7 +9,11 @@ module FlexNBD
 
     def initialize( addr, port, err_msg, source_addr=nil, source_port=0 )
       timing_out( 2, err_msg ) do
-        @sock = TCPSocket.new( addr, port, source_addr, source_port )
+        @sock = if source_addr
+          TCPSocket.new( addr, port, source_addr, source_port )
+        else 
+          TCPSocket.new( addr, port )
+        end
       end
     end