Refactor: move engine_testapp to a separate directory

Change-Id: Id3c7fb98553f9e533da1691ef564005fb0e07fca
Reviewed-on: http://review.couchbase.org/34423
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>