tests: Add regression test for async closures involving HRTBs

I suspect the original code had several issues. The last one that made
the code compile entered `nightly-2024-02-11`. The code fails to build
with `nightly-2024-02-10`:

    $ rustc +nightly-2024-02-10 --edition 2018 tests/ui/async-await/async-closures/unifying-function-types-involving-hrtb.rs
    error[E0277]: expected a `FnOnce(&u8)` closure, found `{coroutine-closure@tests/ui/async-await/async-closures/unifying-function-types-involving-hrtb.rs:31:9: 31:30}`
      --> tests/ui/async-await/async-closures/unifying-function-types-involving-hrtb.rs:31:9
       |
    31 |     foo(async move | f: &u8 | { *f });
       |     --- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnOnce(&u8)` closure, found `{coroutine-closure@tests/ui/async-await/async-closures/unifying-function-types-involving-hrtb.rs:31:9: 31:30}`
       |     |
       |     required by a bound introduced by this call
       |

(Note that you must add `#![feature(async_closure)]` to test with such
old nightlies, since they are from before stabilization of async
closures.)

This was probably fixed by 3bb384a.

Author: Enselic

tests/ui/async-await/async-closures/unifying-function-types-involving-hrtb.rs

@@ -0,0 +1,33 @@
+//! Regresssion test for <https://github.com/rust-lang/rust/issues/59337>.
+
+//@ edition:2018
+//@ check-pass
+
+use std::future::Future;
+
+trait Foo<'a> {
+    type Future: Future<Output = u8> + 'a;
+
+    fn start(self, f: &'a u8) -> Self::Future;
+}
+
+impl<'a, Fn, Fut> Foo<'a> for Fn
+where
+    Fn: FnOnce(&'a u8) -> Fut,
+    Fut: Future<Output = u8> + 'a,
+{
+    type Future = Fut;
+
+    fn start(self, f: &'a u8) -> Self::Future { (self)(f) }
+}
+
+fn foo<F>(f: F) where F: for<'a> Foo<'a> {
+    let bar = 5;
+    f.start(&bar);
+}
+
+fn main() {
+    foo(async move | f: &u8 | { *f });
+
+    foo({ async fn baz(f: &u8) -> u8 { *f } baz });
+}