This Pull Request fixes/closes #718.
It changes the following:
- Adds a new `boa_runtime` crate, that will only include `console` for now
- Changes the `boa_cli` crate to use the new `boa_runtime` crate for the console, instead of the `console` feature of `boa_engine`
- Removes the `console` feature in `boa_engine`
- Adds a new `boa_testing` helper crate with some useful functions for testing `boa`. This part duplicates the code from `boa_engine`, but I could not make `boa_engine` work with this crate as a dependency due to circular dependencies. Maybe doing it a bit generic could work, but didn't have enough time to check it.
To be checked: wether the WASM example works as expected with the console.