[wasm] Sleep awakes to early

[VincentJousse]

The documentation says :

The thread may sleep longer than the duration specified due to scheduling specifics or platform-dependent functionality. It will never sleep less.

But the duration is truncated towards zero.

The std::thread::sleep documentation says :

Platforms which do not support nanosecond precision for sleeping will have dur rounded up to the nearest granularity of time they can sleep for.

Duration should be rounded up to the nearest millisecond.