[SPARK-17920][FOLLOWUP] simplify the schema file creation in test

## What changes were proposed in this pull request?

a followup of https://github.com/apache/spark/pull/19779 , to simplify the file creation.

## How was this patch tested?

test only change

Author: Wenchen Fan <wenchen@databricks.com>

Closes #19799 from cloud-fan/minor.