Free cmdline buffer in engine_testapp

This isn't a huge problem if we don't free this,but it causes
valgrind warnings.

Change-Id: Ia7310261e6623cba8fb326ea7b76e5ab6ec284c0
Reviewed-on: http://review.couchbase.org/34345
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>