MB-11611: Add unit test for file_logger

Change-Id: I1330be0c364eb7310d9f2a060c0e091580a3755f
Reviewed-on: http://review.couchbase.org/39729
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>