Standardize js and sys tests

[syrusakbary]

In the Wasmer API, we have two kind of tests: js and sys ones depending on the feature used.

However, both are testing the Wasmer API (which should be resilient across the js or sys feature).
In general, we should unify the crates/api/tests for both js and sys (and using the test or wasm_bindgen macro depending on the target/feature).
That way we test our that our public API is resilient

[fschutt]

Ideally this should be done after #3129 is merged, but I can start working on it

[syrusakbary]

Closing