Introduce #[test] before the first exercise (Fibonacci)

[mering]

Using cargo's first class testing support is so much nicer than manually cargo run all the time during development. People should get used to writing tests instead of manually writing binaries for testing their code right from the beginning. Luckily it is introduced for the second exercise (Collatz length).

[djmitche]

Students need to see both approaches. Especially in the sandbox, the running a binary crate is pretty common and useful as it shows the output.

As you've observed, #[test] is used in the second exercise, and I think that's close enough to the beginning of the course to highlight both approaches.

[djmitche]

Ah, and I just remembered #1581 which would at least make this consistent. That's assigned to me but I'd be happy to reassign if you're willing to work on it!

[mgeisler]

Using cargo's first class testing support is so much nicer than manually cargo run all the time during development. People should get used to writing tests instead of manually writing binaries for testing their code right from the beginning.

Dustin is right that we need the more primitive "look at the output" approach right now — simply because of how we teach primarily using the embedded Playgrounds.

However, now that we have #2503 merged, which adds support for showing stderr output, we can also look at executing tests! Why, you will ask... because that PR made me realize that there are more advanced Playground API endpoints. We can send a request and tell the Playground to execute tests if we like.

@mering, please let us know if you want to get involved with that.