[SPARK-14914][CORE] Fix Resource not closed after using, for unit tests and example

## What changes were proposed in this pull request?

This is a follow-up work of #15618.

Close file source;
For any newly created streaming context outside the withContext, explicitly close the context.

## How was this patch tested?

Existing unit tests.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #15818 from wangmiao1981/rtest.