Fixed wrong bid value 'multiple-item' End-to-End test
Added 'receives xmpp messages' unittest

test that sniper receives all messages from items.
some xmpp server - embedded vysper, XMPP Server - does not work well.
at most case, they fail to login or receive messages when receiving
requests at same time.
Add work-around code to pass 'receives xmpp messages' unittest

vysper and XMPP Server app does not keep request messages which came at
same time.
to avoid this issue, added delay after login and sendMessage
Added delay to FakeAuctionServer

added delay to login and sending message in FakeAuctionServer.
now, vysper and XMPP Server app fails less.
this commit is workaround.