INT-2880 Fix System.out Concurrent Race Condition

Gradle may run tests using multiple threads.

Using shared resources (e.g. `System.out`) may produce concurrency issues.

* Remove usage of `System.out`
* Add mocking around `LoggingHandler#messageLogger`.

JIRA: https://jira.springsource.org/browse/INT-2880
Merge pull request #705 from artembilan/INT-2880