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 as:

Sometimes we want to reuse some helper functions between tests, but not directly test these helpers. Also, these helpers are meant to be used only by the test suite, so we don't want to include them in our final library/program. To accomplish this, we annotate the helper functions with `#[cfg(test)]`.

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Also I think there should be at least two tests that use the helper function in the example (to reinforce the "reuse" idea). And we should also mention that `#[cfg(test)]` can also be applied to a module.