42711 multiple log messages

- Bump Cocoalumberjack to version 2.0.0-rc
- Set up test logging inside dispatch_once block
I personally don't mind having logs switched on during testing but I don't have a strong opinion either way. Mike's point about async logging is relevant though, and we wouldn't want the performance hit of doing all log sync.

The most important thing is to get rid of the dispatch_once block. So you can either just put removeAllLoggers into initialize or put the CDTChangeLogLevel calls into the relevant once-per-test-run place.