Add some simple unittests + a simple workflow

[learn-more]

These tests are mostly based on the files from https://github.com/protocolbuffers/protobuf/tree/main/src/google/protobuf/testdata .

The current observed behavior is added in the unittest.

@mxrch Please review the unittests to see if the code behaves as expected!

[mxrch]

Thank you so much for your work on this !