Document how to use skylib tests in the repository

[furtib]

Describe the task

Using skylib testing for the first time is very hard; we should help contributors create their first test.

Scope

• Document how to create a skylib analysis and unit test.
• Create a basic (example) skylib test for both analysis and unit test.

Definition of Done

• The creation of skylib tests is well documented and followable.
• An easy (basic) example is provided for new tests to be based on.