various tests related tasks

[Ivshti]

• [x] getLatestMsg - SHOULD RELY ON PROPER TIMESTAMP/SEQUENCE; maybe a primitive DB abstraction?
• [x] https://github.com/AdExNetwork/adex-validator-stack-js/issues/61
• [x] those tests should be more robust - and rely on some feedback messages from the validator stack #61 - perhaps InvalidNewState message
• [x] split tests in multiple files
• [x] decrease repetitiveness in tests (DRY)
• [x] try to decrease waitTime/propagation logs
• [x] tests: go through each one, try to break them by touching the code

[Ivshti]

done in #78