Add a `wasm-pack test` command

[fitzgen]

This still needs some design work before we start any PRs!

• Should run wasm-pack-test on the project, if configured.

  • Depends on Publish a wasm-bindgen-test for use as an internal detail within wasm-pack test wasm-bindgen#615

• Should run npm test on the project, if it contains JS integration tests.

Open questions:

• How to best determine whether a project is using wasm-pack-test? Probably reading Cargo.toml dev dependencies?

• How to best determine whether a project has JS integration tests? Inspect whatever mechanism we settle on for including random stuff into the pkg directory / merging things into package.json?

• Does this run native cargo #[test]s too?

• Does this command install geckodriver / chromedriver for you if needed?