Part of the testing performed in the `console-subscriber` integration
tests is detecting self wakes. This relied upon the `yield_now()` from
Tokio.

However, the behavior of this function was changed in
tokio-rs/tokio#5223 and since Tokio 1.23 the wake doesn't occur in the
task that `yield_now()` is called from. This breaks the test when using
a newer version of Tokio.

This change replaces the use of `yield_now()` with a custom
`self_wake()` function that returns a future which does perform a self
wake (wakes the task from within itself before returning
`Poll::Pending`).

The same custom `self_wake()` is also included in the `app` example so
that it shows self wakes correctly.

Tokio has been updated to 1.28.2 in the lock file (the last with
compatible MSRV) so that this fix is tested.

Ref #512