test: add a test for add command

It isn't passed...

How to reproduce:

    terminal1% DROONGA_CATALOG=sample/cluster/catalog.json fluentd -c sample/cluster/fluentd.conf -vv

    terminal2% test/command/run-test.rb --port 23003
    test/command/suite/groonga/add/array.test: NO RESPONSE

    ==== Test Results ====
    test/command/suite/groonga/add/array.test: no_response

    ==== Summary ====
    {:no_response=>1}

Expected:

The command on terminal2 gets any response.