Fix flaky group_concat() query tests

The ordering of results returned by the  group_concat() tests were not deterministic. This
fixes the problem by switching the test cases to use a subquery with an order by.

Also fixed a similar problem with the limit and union tests.

Change-Id: Ibfe3c1597229cf5156af3a69b26bcce93abe28df
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3822
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>