Replace mockito in GitCommandsExecutorTest

[MarkEWaite]

Replace mockito in GitCommandsExecutorTest

Use real objects instead of mock objects.

Use JUnit 4.13 assertThrows so that try / catch is not needed for exception testing.

Testing done

Confirmed that tests pass on my Linux computer with Java 21. Rely on ci.jenkins.io to test Windows with Java 17.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
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