Make LoggingEvent::getFormattedMessage public

[javathought]

In tests, I want to make assertions on the message that is generated in the logging system, not the template with positional args with "{}".

It could therefore be hard to test arguments, as it should need creating real instance of objects passed as args to logging, not just text really logged

[valfirst]

@javathought you might want to migrate to my fork: https://github.com/valfirst/slf4j-test, the improvement is already released: valfirst#23

[sasuw]

I have also created a fork from the project, where the only change currently is that LoggingEvent.getFormattedMessage is public. See https://github.com/sasuw/slf4j-test