## What changes were proposed in this pull request?
Fair Scheduler can be built via one of the following options:
- By setting a `spark.scheduler.allocation.file` property,
- By setting `fairscheduler.xml` into classpath.

These options are checked **in order** and fair-scheduler is built via first found option. If invalid path is found, `FileNotFoundException` will be expected.

This PR aims unit test coverage of these use cases and a minor documentation change has been added for second option(`fairscheduler.xml` into classpath) to inform the users.

Also, this PR was related with #16813 and has been created separately to keep patch content as isolated and to help the reviewers.

## How was this patch tested?
Added new Unit Tests.

Author: erenavsarogullari <erenavsarogullari@gmail.com>

Closes #16992 from erenavsarogullari/SPARK-19662.