Add some JS code to the wat2wasm test

[fbitti]

When reading the tutorial and visiting wat2wasm for the first time, it was confusing that, by merely copying the wat code the output didn't change, and I suddenly broke the output when I touched the JS code. Providing a full example, even without the promise to explain the details right now:

• makes this initial test on wat2wasm less frustrating
• reinforces what is explained next, that wasm currently works in combination with javascript, by providing this first experience to the reader.

✋ A similar PR may already be submitted! Comment: I checked but it does not exist.

For more information, see the [contributing guide][contributing]. 👫 Comment: I read the contributing guide!