Add unit tests for the `kstring` module

[iWas-Coder]

Describe the feature

There is a need (because of some trouble in #15 dev journey) to add and implement unit tests that covers up all kstring's codebase, to verify that they work as expected.

Why should it be implemented? (optional)

The kstring module is a core component of the engine, thus a lot of parts depend on it; if it does not work as expected, it could create a lot of problems in a wide range of places.

Do you think there is any alternative? (optional)

N/A

Additional context (optional)

N/A

Extra questions (optional)

• I have used the search function to check if an issue already exists
• I'd like to work on this issue