Add parse JSON test in test_tpt_reports.py

[taylornguyenntt]

🗣 Description

This PR resolves issue https://github.com/cisagov/tpt-reports/issues/21 and creates a test case for the parse_json function in tpt_reports.py.

💭 Motivation and context

This will add a test case to an untested function.

🧪 Testing

All automated tests, including this new test, were successful.

✅ 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.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.

[dav3r]

Also, the "Testing" section of this PR's description is blank. You should always include some explanation of how the changes were tested, even it's something simple like "All automated tests, including this new test, were successful."

[coveralls]

Pull Request Test Coverage Report for Build 6316800913

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 6249867391:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls