examples/staging/test: Split and improve chapter

"Testing" chapter has been split in all requested sections. I'm going to discuss other sections I would like to add in #111's thread, but this completes the current TODO of the issue.
I would word this paragraph like this:

Sometimes we want to check that a certain code must trigger a task failure. To achieve this we mark the test function with the `#[should_fail]` attribute. Now, the test passes if and only if a task failure occurs.