[zlatanza] Adding tests

[zezl7]

Description

This PR implements the unit and regression tests, their test coverage, as well as adds tox functionality.

Changes

1. Wrote the three unit and two regression tests based on the given exercise instructions.
2. Ran code coverage on the written tests and uploaded it as coverage-report.pdf.
3. Added the tox.toml and requirements.txt following the examples in the lecture.