Add test cases

[ameliav]

🗣 Description

This PR adds test cases for report_gen() in report_generator.py. Resolves Issue https://github.com/cisagov/tpt-reports/issues/26

💭 Motivation and context

This will add test cases to an untested function and give more coverage.

🧪 Testing

Testing was performed by running PyTest and ensured all added and pre-existing test cases were passing.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [ ] Tests have been added and/or modified to cover the changes in this PR.
• [ ] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Revert dependencies to default branches.
• [ ] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.