[SPARK-22816][TEST] Basic tests for PromoteStrings and InConversion

## What changes were proposed in this pull request?

Test Coverage for `PromoteStrings` and `InConversion`, this is a Sub-tasks for [SPARK-22722](https://issues.apache.org/jira/browse/SPARK-22722).

## How was this patch tested?

N/A

Author: Yuming Wang <[email protected]>

Closes #20001 from wangyum/SPARK-22816.

Author: wangyum