Testing WASM bindings

[fjarri]

WASM bindings can be tested in two ways:

• from the Rust side (see #[wasm_bindgen_test] attribute)
• from the JS side

Do we need only one of those? Both? What JS testing library to use? How to call them from CI?

Testing from the Rust side will increase Rust code coverage. Testing from the JS side is closer to the user experience, but mostly we will be testing how well wasm-pack works, since we don't write any JS code directly. Would be nice to test at least the JS example though.