Fix defragmenter task unit test

Enable defragmenter task if it is disabled by default.

Change-Id: Ie32fe12b13f2095db542ea914eba810260d0fdea
Reviewed-on: http://review.couchbase.org/43460
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>