Implement unit tests

Add the unit tests as provided by @d3-nyarko to the Unit Test project. These tests will ensure the calculations in the software return the expected results to the user.

changed due date to May 04, 2022

changed milestone to % Product Backlog

added Backend Development Sprint 2 labels

assigned to @d2-lorych

created branch 63-implement-unit-tests to address this issue

mentioned in commit 33eba140