update listings 11-01 and 11-03 with add function generated by cargo

[Jbonez87]

This updates the testing code in chapter 11 to reflect what cargo will generate when running cargo new adder --lib.

[matthewjnield]

To add to this, since the testing code generated by cargo automatically includes the line use super::*;, the following paragraph in Chapter 1.1 which describes this line being manually added to Listing 11-6 and the reason for doing so is misleading. Perhaps the paragraph should be moved to Listing 11-1, and altered so that is explaining why the line is (already) there.

Note that we’ve added a new line inside the tests module: use super::*;.
The tests module is a regular module that follows the usual visibility rules
we covered in Chapter 7 in the [“Paths for Referring to an Item in the Module
Tree”][paths-for-referring-to-an-item-in-the-module-tree]
section. Because the tests module is an inner module, we need to bring the
code under test in the outer module into the scope of the inner module. We use
a glob here so anything we define in the outer module is available to this
tests module.

[chriskrycho]

This specific bit was fixed via #3533. I am going to turn your comment into an issue tracking that clarification, @matthewjnield – thanks!