Figure out a way to support checking #[async_attributes::test] in doctests

It would be very valuable for documenting test utils to have `cargo check` be able to run on `#[async_attributes::test]` function in doctests.

For example, the obvious errors here would have been caught, as it clearly can't compile: https://github.com/eaze/preroll/blob/c0d15ef0692ef8f933d3501a708f38f0acd69c9a/src/test_utils/mod.rs#L42-L53 (Wrong naming of `create_app`.)

Unfortunately, `cfg(doctest) seems like it broken and there perhaps is no obvious (or certainly easy) way to fix it, and doesn't seem to be garnering any action: https://github.com/rust-lang/rust/issues/67295

I suggest we find some (even if convoluted) way to run these in doc tests, even if by a manual feature flag.

Start by reproducing how cargo check handles #[async_attributes::test] in doctests, then read the cfg(doctest) discussion in Rust issue #67295. Use the example in src/test_utils/mod.rs as the motivating case and assess a manual feature flag or other route. Done means doctest functions using the attribute are checked and obvious errors such as the wrong create_app name are reported.

Written by the indexing model from the issue text.