Rework Python testing lecture

[IshaanDesai]

The Python testing lecture needs revisions with respect to the following points:

• Add clarification on how assertions are treated when a code is compiled in Release version.
• Use examples when explaining the concepts of unit, integration and regression testing. A simulation code example would be more appropriate than a toy code.
• Make it clear that the assertion statement used at the end of each test (for pytest and unittest) is a Python specific thing and not a general testing practice.
• The slide of test driven development talks about disadvantages of TDD. Sources are necessary here.
• Set code font in orange. Have a look at the automation lecture slides.
• In the code of the demo, use one of the two terms mean and average consistently.

[IshaanDesai]

While using the material for teaching, it has become increasingly evident that the toy code used to explain the concepts of unit, integration, and regression tests is too artificial. A simulation code would make the concepts much more easily understandable.