Tweak file_logger unit test buffer sizes

Increase the buffer size (relative to the expected message length) so
the producer thread isn't blocked waiting for the flusher thread to
timeout (as the flusher high watermark hasn't been reached, yet there
isn't enough space for the producer to write into).

Change-Id: Icea983a841abe1e64858c92cc67eb5bc6b3a805c
Reviewed-on: http://review.couchbase.org/39812
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>