[SPARK-17569][SPARK-17569][TEST] Make the unit test added for work again

## What changes were proposed in this pull request?

A [PR](https://github.com/apache/spark/commit/a6aade0042d9c065669f46d2dac40ec6ce361e63) was merged concurrently that made the unit test for PR #15122 not test anything anymore. This PR fixes the test.

## How was this patch tested?

Changed line https://github.com/apache/spark/blob/0d634875026ccf1eaf984996e9460d7673561f80/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/FileStreamSource.scala#L137
from `false` to `true` and made sure the unit test failed.

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15203 from brkyvz/fix-test.