[SPARK-18872][SQL][TESTS] New test cases for EXISTS subquery

## What changes were proposed in this pull request?
This PR adds the first set of tests for EXISTS subquery.

File name                        | Brief description
------------------------| -----------------
exists-basic.sql              |Tests EXISTS and NOT EXISTS subqueries with both correlated and local predicates.
exists-within-and-or.sql|Tests EXISTS and NOT EXISTS subqueries embedded in AND or OR expression.

DB2 results are attached here as reference :

[exists-basic-db2.txt](https://github.com/apache/spark/files/733031/exists-basic-db2.txt)
[exists-and-or-db2.txt](https://github.com/apache/spark/files/733030/exists-and-or-db2.txt)

## How was this patch tested?
This patch is adding tests.

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

Closes #16710 from dilipbiswal/exist-basic.