## What changes were proposed in this pull request?
This PR extends the existing IN/NOT IN subquery test cases coverage, adds more test cases to the IN subquery test suite.
Based on the discussion, we will create `subquery/in-subquery` sub structure under `sql/core/src/test/resources/sql-tests/inputs` directory.
This is the high level grouping for IN subquery:
`subquery/in-subquery/`
`subquery/in-subquery/simple-in.sql`
`subquery/in-subquery/in-group-by.sql (in parent side, subquery, and both)`
`subquery/in-subquery/not-in-group-by.sql`
`subquery/in-subquery/in-order-by.sql`
`subquery/in-subquery/in-limit.sql`
`subquery/in-subquery/in-having.sql`
`subquery/in-subquery/in-joins.sql`
`subquery/in-subquery/not-in-joins.sql`
`subquery/in-subquery/in-set-operations.sql`
`subquery/in-subquery/in-with-cte.sql`
`subquery/in-subquery/not-in-with-cte.sql`
subquery/in-subquery/in-multiple-columns.sql`
We will deliver it through multiple prs, this is the first pr for the IN subquery, it has
`subquery/in-subquery/simple-in.sql`
`subquery/in-subquery/in-group-by.sql (in parent side, subquery, and both)`
These are the results from running on DB2.
[Modified test file of in-group-by.sql used to run on DB2](https://github.com/apache/spark/files/683367/in-group-by.sql.db2.txt)
[Output of the run result on DB2](https://github.com/apache/spark/files/683362/in-group-by.sql.db2.out.txt)
[Modified test file of simple-in.sql used to run on DB2](https://github.com/apache/spark/files/683378/simple-in.sql.db2.txt)
[Output of the run result on DB2](https://github.com/apache/spark/files/683379/simple-in.sql.db2.out.txt)
## How was this patch tested?
This patch is adding tests.
Author: Kevin Yu <qyu@us.ibm.com>
Closes#16337 from kevinyu98/spark-18871.
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