Track startup time

One important part of Logstash is the time it takes to start. This PR tracks the time between the call to the bin until its able to get the first message.
although more verbose, I think a format like the one below would increase readability

```ruby
[
 "%s"   %           test_name,
 "%s"   %   args[:start_time], 
 "%.2f" %      args[:elapsed],
 "%.2f" % args[:events_count],
"..."
]
```