Feature Requests: Add tests for all functions

[unna97]

Is your feature request related to a problem? Please describe.
Add the tests for all functions & add GitHub action to test them automatically.

Few useful resources:

• https://www.youtube.com/watch?v=Kj1WwpPFr-I
• https://www.youtube.com/watch?v=DhUpxWjOhME
• https://wangonya.com/blog/pytest-parametrize/

Add coverage to check automatically test coverage.
https://coverage.readthedocs.io/en/7.2.3/
https://app.codecov.io/gh/unna97/twarc2sql/new

(- Split this feature into multiple issues?)

[unna97]

For SQL database tests:
https://coderpad.io/blog/development/a-guide-to-database-unit-testing-with-pytest-and-sqlalchemy/

[unna97]

#18 tries to solve this issue