`cargo new` now creates a `src/lib.rs` with a `tests` module by default. I've updated the earlier examples in this doc to reflect this. However, I don't know how we want to approach the "introduction" to idiomatic testing that follows in "the tests module" section. I _think_ it should be broken apart, with the module concept being introduced early on, and the `super` concept being addressed when we hit the `add_two` example. I'd like to get agreement on that being the right approach before I do it though.

I _also_ removed the `#fn main() {}` hidden at the beginning of each example, as these cause Rust Playground to not treat the file as a set of tests that it can run. Removing it _should_ cause Rust Playground to display a "Test >" button in the top left when a user runs the code, which will allow them to see the test runner output.

Without these changes, play.rust-lang.org (as of today) would wrap
our examples in `fn main() {}`. This prevents the user from being able
to easily run the tests.

The narrative flows better if we follow what @steveklabnik is doing in
rust-lang/book#288. Therefore, I completely copied it.

I had used `/tmp/adder` for my previous commits. Flipped over to
`/home/you/projects/adder` for consistency with other parts of testing.md

While the commit message on this one sounds terrible, it's really not so
bad. The issue is that our test runner _expects_ a `fn main() {}` in
code blocks that it'll test, but this code really shouldn't have them.
If it did, then clicking the "play" link in the docs would result in
play.rust-lang.org not treating this code as a test example to be run.