[SPARK-16590][SQL] Improve LogicalPlanToSQLSuite to check generated SQL directly

## What changes were proposed in this pull request?

This PR improves `LogicalPlanToSQLSuite` to check the generated SQL directly by **structure**. So far, `LogicalPlanToSQLSuite` relies on  `checkHiveQl` to ensure the **successful SQL generation** and **answer equality**. However, it does not guarantee the generated SQL is the same or will not be changed unnoticeably.

## How was this patch tested?

Pass the Jenkins. This is only a testsuite change.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14235 from dongjoon-hyun/SPARK-16590.