[SPARK-18872][SQL][TESTS] New test cases for EXISTS subquery (Joins + CTE)

## What changes were proposed in this pull request?

This PR adds the third and final set of tests for EXISTS subquery.

File name                        | Brief description
------------------------| -----------------
exists-cte.sql              |Tests Exist subqueries referencing CTE
exists-joins-and-set-ops.sql|Tests Exists subquery used in Joins (Both when joins occurs in outer and suquery blocks)

DB2 results are attached here as reference :

[exists-cte-db2.txt](https://github.com/apache/spark/files/752091/exists-cte-db2.txt)
[exists-joins-and-set-ops-db2.txt](https://github.com/apache/spark/files/753283/exists-joins-and-set-ops-db2.txt) (updated)

## How was this patch tested?
The test result is compared with the result run from another SQL engine (in this case is IBM DB2). If the result are equivalent, we assume the result is correct.

Author: Dilip Biswal <dbiswal@us.ibm.com>

Closes #16802 from dilipbiswal/exists-pr3.