Add Testing

Write automated tests for various functions to... test stuff!

A collaborator and I wrote testing code in our other repository:
https://github.com/Bixkitts/pdf-from-scratch/

I'm quite happy with that simple implementation we have,
I'd like to see something similar in this library perhaps.

This would make a good first issue!
You only need to
add another optional binary to the build, or a "run_tests()" function that conditionally compiles.
This should then utilize various functions of the library and verify the results.

A bonus would be integrating this into CI/CD/automation, but that's completely unnecessary.

Review the library's existing build configuration and the functions that should be exercised, then compare the testing approach in the linked pdf-from-scratch repository. Done means an optional test binary or conditional run_tests() entry point invokes representative library functions and verifies their results; CI integration is explicitly optional.

Written by the indexing model from the issue text.