[SPARK-14833][SQL][STREAMING][TEST] Refactor StreamTests to test for source...

[SPARK-14833][SQL][STREAMING][TEST] Refactor StreamTests to test for source fault-tolerance correctly.

## What changes were proposed in this pull request?

Current StreamTest allows testing of a streaming Dataset generated explicitly wraps a source. This is different from the actual production code path where the source object is dynamically created through a DataSource object every time a query is started. So all the fault-tolerance testing in FileSourceSuite and FileSourceStressSuite is not really testing the actual code path as they are just reusing the FileStreamSource object.

This PR fixes StreamTest and the FileSource***Suite to test this correctly. Instead of maintaining a mapping of source --> expected offset in StreamTest (which requires reuse of source object), it now maintains a mapping of source index --> offset, so that it is independent of the source object.

Summary of changes
- StreamTest refactored to keep track of offset by source index instead of source
- AddData, AddTextData and AddParquetData updated to find the FileStreamSource object from an active query, so that it can work with sources generated when query is started.
- Refactored unit tests in FileSource***Suite to test using DataFrame/Dataset generated with public, rather than reusing the same FileStreamSource. This correctly tests fault tolerance.

The refactoring changed a lot of indents in FileSourceSuite, so its recommended to hide whitespace changes with this - https://github.com/apache/spark/pull/12592/files?w=1

## How was this patch tested?

Refactored unit tests.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #12592 from tdas/SPARK-14833.