Finish documenting whole testui package

[paulsgrudups]

This PR contains changes to add docstrings to the whole testui package to provide documentation for the end-user and improve the linter score.

Small improvements were also made to improve the linter score like:

1. Code styling
2. Line length limit wrapping
3. __contains__ conditions rewritten to {value} in format
4. Early function returns in places where within if else statements both scenarios return from function

[alvarolaserna]

Can we add pylint job into the github actions?

[TDL-EdgarsEglitis]

It's probably worth checking whether the new return calls in testui_driver.py don't break anything.

[TDL-EdgarsEglitis]

Would this fully cover the implementation of #32?

[alvarolaserna]

I think so, but we need to add a pylint job to the pipeline to keep things well formatted

[TDL-EdgarsEglitis]

I suppose the remaining Pylint warnings should also be fixed eventually, but for now this looks good.