migrate fileserver unit tests to pytest

[MKLeb]

What does this PR do?

Migrates fileserver unit tests to pytest

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

Commits signed with GPG?

Yes