[CI/CD] Setup Github Actions to automatically run unittest with each commit

[johnwangwyx]

Unittests are currently in ./test directory and can be run by:

python -m unittest discover -s "./test" -t "./" (i.e running tests in ./test from project's root)

We can setup a single GitHub Action to run the tests on Mac, Windows, and Linux (The three supported OS) with runs-on: ${{ matrix.os }}.