Add additional tests for string lcb_cntl()

Change-Id: I13c9c62c5723a68d49d3f899c77f3bba57375d37
Reviewed-on: http://review.couchbase.org/37804
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>