The coverage-helper::test doesn't work with tokio::test

[Phoenix500526]

Hi, @taiki-e.

Currently, the implementation of coverage_helper replaces std::test with coverage_helper::test. However, this approach doesn't work for tokio::test. Given that tokio::test is widely used, shall we support tokio::test in this crate?

IMO, we can provide a tokio module in coverage_helper, like:

mod test{
    use coverage_helper::{tokio, test};
    #[test]
    fn test_foo() {...}

    #[tokio::test]
    async fn test_bar() {...}
}

What do you think? If this approach is feasible, I can do some contributions.

[taiki-e]

As said in taiki-e/cargo-llvm-cov#273, tokio::test refers ::core::prelude::v1::test, so it's not working with coverage_helper.

The most desirable fix is to change libtest so that the test attribute gets no_coverage, and the second most desirable is to make the change on the tokio side.

[taiki-e]

This problem may have been resolved by tokio-macros 2.3.0, which includes tokio-rs/tokio#6497.