Upgrade wasm-bindgen-test to std::future

[najamelan]

Motivation

Work has been done to start enabling async/await, but the wasm-bindgen-test macro still requires users to return a futures 0.1 future from their test functions. I think it's time we allowed returning a std future.

Proposed Solution

Potentially behind the same futures_0_3 feature as with wasm-bindgen-futures. The relevant code seems to be here.

Alternatives

Wait until it stabelizes completely

[fitzgen]

I think we want to wait until it stabilizes completely. We've historically put a lot of effort into getting our toolchain on stable rustc, and I wouldn't want to regress that without extremely strong motivations.

[alexcrichton]

Oh oops sorry I meant to reply to this! I think that we'll want to hold off on changing #[wasm_bindgen_test(async)] until futures are stable, but like with opt-in support for libstd's Future trait I think we could take a strategy like #[wasm_bindgen_test(async_std)] or something like that which means "use std::future::Future" and couple that with a feature of the wasm-bindgen-test crate and we should be able to implement this today without too much impact.

[Pauan]

Future will be available on Stable on July 4th, so I'm not a fan of short-term solutions.

[ctaggart]

moved comment to #1695 (comment)

[alexcrichton]

I've posted an initial version of this to #1741