[MINOR] [SQL] Re-enable `explode()` and `json_tuple()` testcases in ExpressionToSQLSuite

## What changes were proposed in this pull request?

Since [SPARK-12719: SQL Generation supports for generators](https://issues.apache.org/jira/browse/SPARK-12719) was resolved, this PR enables the related testcases: `explode()` and `json_tuple()`.

## How was this patch tested?

Pass the Jenkins tests (with re-enabled test cases).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12329 from dongjoon-hyun/minor_enable_testcases.