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 think this chapter could start like this:

Rust provides basic infrastructure for unit testing, where a test unit is a function with signature `fn () -> ()`. To mark a function as a unit test, annotate it with the `#[test]` attribute. If a task failure occurs when the test function is called, the test has "failed", otherwise the test has succeeded.

(Maybe the last sentence should go in the next paragraph?)