Add unit tests

[bkeryan]

Tech Debt

This project doesn't have any unit tests yet.

In addition to the tests themselves, it also needs:

• GitHub Actions workflow to run them and report results. https://github.com/ni/nitypes-python/blob/main/.github/workflows/run_unit_tests.yml and https://github.com/ni/nitypes-python/blob/main/.github/workflows/report_test_results.yml are a good starting point.
• Update the branch protection ruleset's required checks to include the unit tests, so that PRs cannot be merged when the tests are failing.

AB#3789761

[tjying95]

Unit tests implemented in #64