CBQE-1622: Modified test_append test for value_size and loop_count

Change-Id: Iaddd79a6251cc13c9a3d05db4002fb90e0775d5c
Reviewed-on: http://review.couchbase.org/32902
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>