PR: tests/unit/test_diff: increase test coverage

[bigtedde]

Description

There are some areas of opportunity in the diff_result module for increased test coverage of some class methods and functions. The test_diff module is already well parametrized and does not have a notable amount of code duplication, so here I would like to focus mainly on increasing code coverage.

Related Issue

#1754

Motivation and Context

One of my Google Summer of Code projects involves cleaning up the Vorta tests, reducing duplicated code, and increasing code coverage.

How Has This Been Tested?

Tests work locally by running pytest, and they pass the GitHub CI checks.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.