Async / tokio::test macro Support?

[leeola]

Not sure how tightly this library is coupled to proptest or proptest_derive, but perhaps related to proptest-rs/proptest#179 it would be great if we could use this with async tests.

[frozenlib]

#[test_strategy::proptest] expands to proptest::proptest! { ... }, so if proptest! { } supports async fn, test_strategy will also be able to support async fn.

[frozenlib]

I have experimentally implemented support for async fn in fd49486.

You can write tests for async fn by adding a dependency on tokio to Cargo.toml and adding async = "tokio" to the argument of #[test_strategy::proptest] as shown below.

[dev-dependencies]
proptest = "1.1.0"
tokio = { version = "1.28.1", features = ["rt-multi-thread"] }
test-strategy = { git = "https://github.com/frozenlib/test-strategy.git" }

#[test_strategy::proptest(async = "tokio")]
async fn test() {
	// todo
}

The above code generates the following code.

proptest::proptest! {
    #[test]
    fn test() {
        let ret: ::core::result::Result<_, ::proptest::test_runner::TestCaseError> =
            tokio::runtime::Runtime::new()
                .unwrap()
                .block_on(async move {
                    {
                    	// todo
                    }
                    Ok(())
                });
        ret?;
    }
}

I am not sure if this is the right way to do it, as block_on will be called repeatedly, but I believe that if we try to do more than this, we will need to support async fn by proptest::proptest!{ }.