Save signing log

We are collecting data while signing transactions at Pheix::Model::Database::Blockchain via $.sgnlog is default([]) is rw.

But this log never saved on testing, so I propose to add flush_signing_session() method Pheix::Test::BlockchainComp::Helpers, it will:

1. take $dbobj and $filename as input arguments
2. save $!sgnlog into file
3. clean $!sgnlog on successful save — No a good idea to modify logs silently, if you want to clean log do it manually in caller on successful log save via $dbobj.chainobj.sgnlog = [];.