add regression test for nested selects using UNION ALL and multiple aliases of the same column.

Pre 7.x code would throw a NPE with a query similar to [1]. Adding JUnit to prevent regression

[1]
SELECT enterprise_id FROM (
SELECT * FROM (
SELECT id, id AS display_id, 103 AS enterprise_id FROM (
SELECT 'x' as id) AS nexted103) as table103Source
UNION ALL
SELECT 'x', 'x' AS display_id, 101 AS enterprise_id FROM (
SELECT 'x') AS nested101
UNION ALL
SELECT 'x', 'x' AS display_id, 100 AS enterprise_id FROM (
SELECT 'x') AS nested100) as tableFrom1stSelect
WHERE enterprise_id = 100