Added unit tests for file package

[tdawe]

Description

Targets the usr/spark/unit-test-improvements branch, not main.

Code coverage above 90% for this package:

ok      github.com/dell/csi-powermax/v2/pkg/file        0.039s  coverage: 97.9% of statements

Added unit tests for file package

GitHub Issues

List the GitHub issues impacted by this PR:

GitHub Issue #

Checklist:

• Have you run format,vet & lint checks against your submission?
• Have you made sure that the code compiles?
• Did you run the unit & integration tests successfully?
• Have you maintained at least 90% code coverage?
• Have you commented your code, particularly in hard-to-understand areas
• Have you done corresponding changes to the documentation
• Did you run tests in a real Kubernetes cluster?
• Backward compatibility is not broken

How Has This Been Tested?

Ran unit tests