Add unit tests for the tempest.common.utils.file_utils

This commit adds a positive and negative unit tests for the single
method from file_utils in tempest.common.utils.

Partially implements bp unit-tests

Change-Id: Ic19428a10785afd8849442f4d1f8f8e0a87f549b
Merge "Add unit tests for the tempest.common.utils.file_utils"