testapp: Add option to run memcached under Valgrind

Change-Id: Ice7dd44fc76a4b5771551da7a16c5dd9640750d5
Reviewed-on: http://review.couchbase.org/38558
Tested-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>